Many of Burundi's opposition figures have gone into hiding after the killing of Zedi Feruzi, leader of the Union for Peace and Development party, a major opposition said.
Agathon Rwasa said a number of politicians, journalists and civic activists are hiding in Bujumbura, the capital, because they do not feel safe after the shooting on Saturday night .
"The life of anyone who opposes (President) Pierre Nkurunziza is in danger...people are being assassinated here and there," he told the Associated Press news agency.
One of Feruzi's bodyguards was also killed in the drive-by shooting.
At least 20 people have died in street protests in Bujumbura that have gone on for four weeks since the announcement that Nkurunziza would stand for a third term in the June 26 elections.
At least 100,000 Burundians have fled to neighboring countries fearing political violence ahead of the elections.
The protests boiled over on May 13 when an army general announced a coup, which was crushed in 48 hours by others in the army loyal to the president.
Nkurunziza was in Tanzania attending a regional heads of state summit discussing the crisis in Burundi at the time of the coup.
He returned to Bujumbura cautiously days after the failed coup attempt, and has since linked the street protests and the coup attempt.
Four independent radio stations and TV station have been attacked with grenades and at least one has been completely burned, leaving the state-owned media as the only source of information for many Burundians who don't leave in the capital.
Most journalists who witnessed the attack say perpetrators were government intelligence officials and police.