A former Islamist militant in Somalia now running for a regional presidency has been arrested and beaten, precipitating clashes between supporters and security forces, his spokesman and witnesses said on Thursday.

South West will be the first of Somalia’s seven semi-autonomous regions to hold presidential elections in the coming months, a critical juncture in a growing power struggle between the U.S.-backed central government and regions where militants retain a presence following a long civil war.

"Candidate Sheikh Mukhtar (Robow) was just beaten and arrested by Ethiopian (peacekeeping) forces in Baidoa now," said Muawiya Mudeey, a spokesman for Robow.

"First he was called in by the interim president of South West state and when he reached that office, Ethiopian forces beat him terribly and they arrested him. And now there is fighting between residents and government forces in Baidoa."

Baidoa residents said that fighters allied to Robow had clashed with Ethiopian and central government forces at a checkpoint, while phone links with the city had become patchy.

"There are casualties. We see Ethiopian tanks being moved into the town. Now there is sporadic gunfire. Tension is very high now and all shops are closed," Ahmed Abdullahi, a Baidoa shopkeeper, told Reuters.

Mohamed Aden, a local elder, said Ethiopian and local Somali security forces participated in Robow's arrest. "We do not know the reason but we think it is because of politics," Aden said.

"...Anything can happen. I mean, war can start anytime."

Regional officials in Baidoa, the state capital of South West, could not immediately be reached for comment. There was also no immediate comment from Ethiopian officials.

Robow, a former prominent al Shabaab insurgent and group spokesman, lay low for several years before publicly renouncing violence and recognising the authority of the federal government in the capital Mogadishu in August 2017.

The Mogadishu government tried to bar his presidential candidacy in South West because of remaining U.S. sanctions against him. But the state electoral commission last month dismissed Mogadishu's demands and accepted his candidacy.

However, on Dec. 1 the commission postponed South West's vote for the third time, saying it was not sufficiently prepared amid lingering tensions with Mogadishu.

Somalia has been trying to claw its way out of the embers of the civil war that engulfed it in 1991, when clan warlords overthrew a dictator and then turned on each other.

Al Shabaab has sought for over a decade to topple the weak central government in Mogadishu and implement its strict version of Islamic law. It was driven out of the capital in 2011 but retains a strong presence in some areas including South West.