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The throng of Central American migrants advancing toward the U.S. border in southern Mexico has now swelled to a whopping 5,000 people as they defy the efforts of four governments to break them up. 

Thousands of mostly Honduran migrants rose at dawn on Sunday from the shores of a river between Guatemala and Mexico and continued their trek northward, overwhelming Mexican government attempts to stop them at the border.

Their numbers swelled from 2,000 to about 5,000 overnight and at first light they set out walking toward the Mexican town of Tapachula, 10 abreast in a line stretching approximately a mile.

Several hundred more already had applied for refugee status in Mexico and an estimated 1,500 were still on the Guatemalan side of the Suchiate River, hoping to enter legally. 

It came as President Donald Trump fired off a tweet on Sunday afternoon, saying: 'Full efforts are being made to stop the onslaught of illegal aliens from crossing our Southern Border. 

'People have to apply for asylum in Mexico first, and if they fail to do that, the U.S. will turn them away. The courts are asking the U.S. to do things that are not doable!' 

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The throng of Central American migrants advancing toward the U.S. border in southern Mexico swelled to a whopping 5,000 people on Sunday as they defy the efforts of four governments to break them up

Aerial view of Honduran migrants heading in a caravan to the US, on the road linking Ciudad Hidalgo and Tapachula, Chiapas state, Mexico

A Mexican Federal Police officer speaks with Honduran migrants heading in a caravan to the US, on the road linking Ciudad Hidalgo and Tapachula, Chiapas state, Mexico, on Sunday

Mexican Federal Police officers are deployed as thousands of Honduran migrants resumed their march toward the United States on Sunday 

Honduran migrants are seen above on the road linking Ciudad Hidalgo and Tapachula, Chiapas state, Mexico, Sunday

Honduran migrants taking part in a caravan heading to the US, cross the Suchiate River, natural border between Guatemala and Mexico, in makeshift rafts on Sunday

Trump went on to add in a separate tweet: 'The Caravans are a disgrace to the Democrat Party. Change the immigration laws NOW!'

It was not immediately clear where the additional travelers had materialized from since about 2,000 had been gathered on the Mexican side Saturday night. 

They seemed likely to be people who had been waiting in the Guatemalan town of Tecun Uman and who decided to cross during the night. 

The migrants pose a challenge to Mexican President Enrique Pena Nieto's promise late last week to stop the travelers' plans to press ahead to the U.S. border without the proper documents. 

As the U.S.-bound caravan moved into Mexico, Mexican police in riot gear were deployed at highway junctions and a military helicopter circled overhead, prompting many migrants to wonder if police will make mass arrests and seek to turn the caravan back.

Many of the bedraggled travelers of the swollen caravan appeared determined to keep moving, saying they are fleeing a toxic mix of violence, poverty and endemic corruption and would not turn back.

The multitude of migrants clogged the highway leading north from the border city of Ciudad Hidalgo in the southern Mexican state of Chiapas, many breaking into song.

Many of the bedraggled travelers of the swollen caravan appeared determined to keep moving, saying they are fleeing a toxic mix of violence, poverty and endemic corruption and would not turn back

Their numbers swelled from 2,000 to about 5,000 overnight and at first light they set out walking toward the Mexican town of Tapachula, 10 abreast in a line stretching approximately a mile

Honduran migrants wait to cross the border from Ciudad Tecun Uman, Guatemala, to Ciudad Hidalgo, Mexico on Sunday

A police officer looks on as Central American migrants walk along the highway near the border with Guatemala, while they continue their journey trying to reach the U.S.

Parents carrying their children were among the migrants taking part in the caravan on Sunday as they cross the border from Tecun Uman, Guatemala, to Ciudad Hidalgo, Mexico

A Honduran migrant heading in a caravan to the US, carries a baby on the road linking Ciudad Hidalgo and Tapachula, Chiapas state, Mexico

A baby taking part in a migrant caravan heading to the US, rests on the road linking Ciudad Hidalgo and Tapachula, Chiapas state, Mexico, on Sunday

The slow pace of legal processing prompting several thousands to cross the river illegally by raft or swimming, according to local officials and migrant organizers

'If you send us back, we will return!' a large crowd shouted in unison under the intense glare of the morning sun.

'We are not criminals, we are workers!' 

U.S. President Donald Trump has threatened to halt aid to Honduras and Guatemala, and potentially close down the U.S. border with Mexico with the help of the military if the migrants' march is not stopped.

Mexico's government has said throughout the past week that it would register the migrants and process requests for asylum. Those attempting to skip the process would face deportation, but the size of the caravan will test Mexico, which has sought help from the U.N. to manage the issue.

Encamped for two nights using backpacks for pillows and tents made of trash bags on a long bridge between Guatemala and Mexico, the migrant caravan began in San Pedro Sula, Honduras, last week and grew exponentially as it passed through Guatemala.

On Saturday, Mexican immigration authorities only allowed some 640 migrants through the official border crossing on a bridge spanning the Suchiate River.

The slow pace of legal processing prompting several thousands to cross the river illegally by raft or swimming, according to local officials and migrant organizers. 

Thousands of Honduran migrants resumed their march toward the United States on Sunday from the southern Mexican city of Ciudad Hidalgo

Migrants, mostly Honduran, formed a march that stretched over a mile as they attempted to cross into Mexico on Sunday

Thousands were left stranded on a border bridge on Saturday as women and children were allowed into Mexico

A Honduras migrant mother and her son walk across the bridge after crossing the border between Guatemala and Mexico, in Ciudad Hidalgo, Mexico on Saturday

Migrants tired of waiting to cross into Mexico jumped from a border bridge into the Suchiate River, in Tecun Uman, Guatemala, on Friday 

On Sunday, federal police monitored the caravan's progress from a helicopter and had a few units escorting it. Outside Tapachula about 500 federal police briefly gathered along the highway on buses and in patrol units, but officers said their instructions were to maintain traffic on the highway not stop the caravan. They moved on toward Tapachula before the caravan reached them. 

As they passed through Mexican villages on the outskirts of Ciudad Hidalgo on Sunday, the migrants drew applause, cheers and donations of food and clothing from Mexicans.  

Maria Teresa Orellana, a resident of the neighborhood of Lorenzo handed out free sandals to the migrants as they passed. 'It's solidarity,' she said. 'They're our brothers.'

In the tropical heat, Besi Jaqueline Lopez of San Pedro Sula carried an improbable stuffed polar bear with a winter cap, the favorite - and only - toy of her two daughters, Victoria 4 and Elisabeth, 3, as they trudged beside her, all covered in sweat.

A business administration graduate, Lopez said she couldn't find work in Honduras. She wants to reach the USA but would stay in Mexico if she could find work here. 'My goal is to find work for a better future for my daughters,' she said. Her husband, David Martinez, said they were tired, but had to push on to reach their goal of making it to the U.S.

Olivin Castellanos, 58, a truck driver and mason from Villanueva, Honduras, said he took a raft across the river after Mexico blocked the bridge. 'No one will stop us, only God,' he said. 'We knocked down the door and we continue walking.' He wants to reach the U.S. to work. 'I can do this,' he said, pointing to the asphalt under his feet. 'I've made highways.'

Dozens of Honduran migrants crossed the Suchiate River, which separates Guatemala from Mexico, on foot and boats

Immigrants are seen arriving in Ciudad Hidalgo, Mexico on Saturday after crossing the border from Guatemala

The caravan of thousands of Central Americans, mostly from Honduras, hopes to eventually reach the United States 

Thousands of migrants made their way through Guatemala's northwestern border and flooded onto a bridge leading to Mexico, where riot police battled them back. Others made their way through the water