The former No 2 leader of Somalia's al-Shabab extremist group, who is now a top candidate in a regional election next week, was arrested on Thursday, prompting violent protests in which four people were killed, officials said.
The arrest could re-ignite old tensions between Somalia and neighbouring Ethiopia.
Robow was flown from Baidoa to the capital, Mogadishu, said a Somali intelligence official, who spoke on condition of anonymity because he was not authorised to speak to the media.
In a statement, Somalia's security ministry confirmed Robow's arrest, citing the federal government's earlier ban on his candidacy and alleging that he had failed to renounce extremist ideology.
The statement also accused Robow of mobilising armed forces to threaten the security of Baidoa.
There was no immediate comment from Ethiopia or the AU force, AMISOM.
Robow made a high-profile defection last year from al-Shabab, Africa's most active extremist group, and is now running to be regional president of Southwest state in elections scheduled for December 19.
His defection was welcomed by Somalia's government, but not his candidacy, which appeared to take some officials by surprise.
The Ethiopian soldiers arrested Robow at the regional president's residence in Baidoa, according to witnesses.
The regional candidates had been summoned for a meeting there. Somali police accompanied Ethiopian soldiers in the arrest, officials said.
Gunfire erupted in Baidoa as Robow's supporters protested his arrest. Armed militias thought to be linked to Robow were seen on the streets, said police Col Ahmed Abdi.
At least four people were killed, including an Ethiopian soldier, in the clashes between troops and armed men thought to be Robow's supporters, Abdi said.
The Ethiopian army deployed tanks and hundreds of soldiers in Baidoa following the fighting, he said.
The lanky, bearded Robow last year had a USD 5 million bounty on his head, offered by the US government, but the reward was dropped before he defected from al-Shabab.
Although he is blamed for directing much of the rebels' violence, he is generally popular in the Southwest region.
Robow's controversial campaign exposed the rift between Somalia's federal government based in Mogadishu and the regional government.
Worried about Robow's popularity, the federal government had sent at least one high-level official to try to persuade him to step aside.
"I was asked to relinquish my candidacy but I'm telling you that ... I will be running for president," Robow told supporters in October.
"With the help of God, we will win and peace will win."
The US military's Africa command, which carried out operations against al-Shabab alongside Somali and AU forces, said it was aware of the reports of Robow's arrest.
The arrest was criticised by Rashid Abdi, an analyst with the International Crisis Group.
"A thoroughly daft move on part of the Ethiopians," said Abdi in a tweet.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)