A prominent Burundi human rights activist has been shot and seriously wounded by gunmen on motorbikes, his family and witnesses say.

Pierre Claver Mbonimpa was attacked in the capital, Bujumbura.

He was a stern critic of President Pierre Nkurunziza's controversial bid for a third term in office.

On Sunday presidential aide Gen Adolphe Nshimirimana was killed in an attack on his car in Bujumbura. He was in charge of the president's personal security.

Burundi has suffered months of unrest since President Nkurunziza's decision to seek a third term

Burundi has suffered serious unrest since President Nkurunziza's decision in April to seek a third term in office.

Opponents argued this contravened the constitution and there was a failed coup attempt in May.

A presidential election was then held last month which Mr Nkurunziza won, but which was boycotted by the opposition.

Bikers with guns

A relative of Mr Mbonimpa told the Reuters news agency that the activist was in intensive care in a Bujumbura hospital following the shooting which took place near his home north of the capital on Monday evening.

"He was going home when suddenly a group of motorcyclists encircled him and started shooting at him. Many of the bikers had guns," the relative said.

Mr Mbonimpa is believed to be one of the few members of Burundi's civil society opposed to the president who had not fled the country.

He is reported to have been jailed several times because of his activism and in April was held without charge for more than 24 hours by the intelligence services after he called for protests against the president's efforts to secure a third term.

Correspondents say that Mr Mbonimpa's work over the years for the rights of prisoners and others has won international acclaim and that chilling rumours of his death had circulated in Burundi before the shooting took place.