Leading Burundian dissident Hussein Radjabu has escaped from a prison where he was serving a 13-year term for plotting against the state.
Mr Radjabu, a former head of the governing party, reportedly escaped with three prison officers.
He was widely regarded as Burundi's most powerful man until his arrest in 2007, amid a rumoured rift with President Pierre Nkurunziza.
Tension has been growing over the president's plans to seek a third term.
The governing CNDD-FDD party is said to be split between those who support Mr Nkurunziza's intention to stay in office, and those who oppose it.
Mr Radjabu's escape was aided by at least three guards, including the chief warden in charge of the prison's security, a police spokesman told AFP news agency.
At least two other inmates escaped with him into vehicles that had been waiting outside the prison, the spokesman said.
The escape reportedly took place before midnight on Sunday evening, at a time when many guards were asleep.
In 2005, Mr Nkurunziza, a former Hutu rebel leader, became the first president to be chosen in democratic elections since the start of Burundi's civil war in 1994.
Mr Radjabu, who was also a rebel leader, became the head of the CNDD-FDD at the same time. He was regarded as the real power in the country.
Many Burundians saw his arrest in 2006 - on charges of plotting against state security - as politically motivated.
Mr Nkurunziza was re-elected in June 2010. The vote was boycotted by the opposition, which complained of fraud in the earlier local elections.
Opposition leaders and international observers have since complained of a growing crackdown on opposition parties and the media.