A grenade attack killed two people and wounded at least 28 in Rwanda's capital Kigali during the Friday evening rush hour, the police said on Saturday.

Police spokesman Theos Badege said four people had been arrested in connection with the blast and investigations were underway. No details of the suspects were given.

The grenade exploded near a busy bus terminal in Giporoso, Remera, a suburb of Kigali.

Badege said the police cannot at the moment figure out the motive of those who threw the grenade, but were not ruling out that the attack was designed to cause instability.

So far two people are dead and among the 28 who sustained injuries, 10 are critically injured and hospitalised, Badege told Reuters.

This particular type was a hand grenade. It went off at around 7 p.m. Kigali time at Giporoso near the bus park.

It could be an isolated incident, but we can't say it was by accident because someone was definitely moving with this grenade. A mini bus was also party damaged, he said.

Kigali was hit by a string of grenade attacks last year which the government has blamed on two high-ranking officers now in exile.

One of the exiles -- Lieutenant-General Faustin Kayumba Nyamwasa, a former chief of staff and ambassador to India -- denied the allegations last year.

He said the Rwandan authorities had staged grenade attacks and then accused him of being behind them.

Nyamwasa now lives in South Africa. He was shot and wounded last June in an attack his wife blamed on Rwanda, a charge labelled as preposterous by Kigali.

Many grenades were left over from lengthy conflicts in the Great Lakes region and are sometimes used to settle scores.

While there has generally been little crime in recent years in the central African country -- where 800,000 were killed in 100 days in the 1994 genocide -- there are occasional bombings.