Samia Suluhu Hassan has become Tanzania's first female leader after her predecessor, John Magufuli, was pronounced dead on Wednesday.
Ms Hassan, 61, was sworn in as president at State House, the government offices in Dar es Salaam, the country's largest city, on Friday.
The former vice president, who studied at Manchester University, was wearing a hijab and holding a Koran in her right hand, as she vowed to uphold the constitution of the East African country.
She joins a short list of women in Africa to have held their country's highest office. The only other at the moment is Ethiopia's President Sahle-Work Zewde, though the role there is largely ceremonial.
Ms Hassan's inauguration comes two days after she announced the death of Mr Magufuli, who had not been seen in public for more than two weeks.
A fierce COVID-19 sceptic, Mr Magufuli said the illness could be prayed away from the country - before finally acknowledging the threat it posed weeks before his death.
It has been speculated that the 61-year-old succumbed to the illness himself, a theory advocated by exiled opposition leader Tundu Lissual, though officially he died of heart failure.
In her first public address as president, Ms Hassan announced 21 days of mourning for Mr Magufuli and two separate public holidays next week.
One will be on Thursday, when the late president will be buried.
Ms Hassan called for national unity following her inauguration. The ceremony was held indoors, and no-one wore a face mask to protect against COVID-19.
"It's not a good day for me to talk to you because I have a wound in my heart," she said.
"Today I have taken an oath different from the rest that I have taken in my career. Those were taken in happiness.
"Today I took the highest oath of office in mourning."
Among the first tasks facing Ms Hassan will be a decision on whether to procure COVID-19 vaccines.
Under her predecessor, the government said it would not obtain any vaccines until the country's own experts had reviewed them.
Despite being vice president since 2015, and having served as a state minister in the previous government, little is known about Ms Hassan's private life.
Born in Zanzibar, the semi-autonomous islands off the coast of mainland Tanzania, in 1960, she studied public administration, first in Tanzania and then as a post-graduate at Manchester University.
In 1978, she married Hafidh Ameir, who is known to be an agricultural academic but has also kept a low profile.
Since Ms Hassan became vice president, the two have not been pictured together.
They have four children. One, Mwanu Hafidh Ameir, is currently a member of Zanzibar's House of Representatives.
