Two people have been killed in a grenade explosion in Rwanda's capital, Kigali, police have said.
Sixteen others were wounded by the blast near a market in Gasabo district.
The security forces have blamed previous grenade attacks on the FDLR Hutu Rwandan rebels in neighbouring Democratic Republic of Congo.
Leaders of a new opposition party in exile, who were also accused of having links to grenade blasts in 2010, condemned Tuesday night's explosion.
"Either the government is implicated in these periodic grenade attacks, so as to find pretext to crack down on the political opposition at a time of dwindling internal and foreign support, or the regime has lost the ability to protect its citizens," Theogene Rudasingwa, of the opposition Rwanda National Congress (RNC), said in a statement.
Mr Rudasingwa, who was once a close adviser to President Paul Kagame and ex-secretary general of the Rwandan Patriotic Front, the rebel army that put a stop to the 1994 genocide, called it a "cowardly act".
He is now based in the US and was tried in absentia last year, along with three other former top RPF aides who accuse Mr Kagame of being intolerant to dissent, and found guilty of threatening state security and propagating ethnic divisions.
"An investigation has been opened but no suspects have been arrested," AFP news agency quotes police spokesman Theos Badege as saying.
More than 20 other people are awaiting trial accused of being behind several other blasts since 2010, mainly in Kigali.
Following the other attacks, the government said the RNC was waging an organised campaign to destabilise the country.
Some 800,000 Tutsi and moderate Hutus were killed in 100-day genocide in 1994.
Afterwards Hutu militias, who had orchestrated much of the killing, fled to eastern DR Congo, where some of them set up the FDLR group.