At least four people have been killed after angry crowds protesting the killing of a popular Muslim preacher with three of his companions rioted in the Kenyan city of Mombasa.

The apparent assassination of Sheikh Ibrahim Omar follows last month's attack on a Nairobi shopping mall claimed by Somalia's al-Shabab group. Associates of Omar said he was the latest victim in a string of extra-judicial killings of Muslims by security forces, an allegation denied by the police.

Rioters set fire to a church in the area on Friday, Reuters news agency reported. Police fired tear gas to disperse the crowd.

Al Jazeera's Andrew Simmons, reporting from Nairobi, said the fatality appeared to suggest that live ammunition was being used during the violence but that was uncomfirmed.

"There's a considerable amount of tension in the coastal city of Mombasa now," Simmons said.

"It has to be said that the Somali community in a large district of Nairobi, Eastleigh, have been calm. Fears of a backlash have not happened there. But, in Mombasa, it is a volatile atmosphere."

Simmons said that latest reports suggested police had the situation under control but that was unconfirmed.

Controversial preacher

Sheikh Ibrahim Omar preached at Mombasa's Masjid Musa mosque, which attracts some radical followers. He was viewed as the successor to Aboud Rogo Mohammed, a controversial preacher accused of links to Somalia's al-Shabab fighters, who was himself assassinated in a drive-by shooting in August 2012.

"They were five people in a vehicle when it was sprayed with bullets," a senior police officer said on Friday from the scene, adding that one passenger survived.

An AFP news agency reporter at the scene said the car, the windows smashed and the doors riddled with bullet holes, had veered off the main coastal highway running north out of Mombasa.

Al-Shabab's deadly assault on the upmarket Nairobi shopping mall killed at least 67 people in a four-day bloodbath. The armed group has threatened to carry out further attacks if Kenya does not pull its troops out of Somalia.

Source: Al Jazeera and agencies