Odinga was in his office meeting the Chief Justice, Willy Mutunga, when police asked everyone to evacuate the building to allow for a search after the grenade was discovered.

Nairobi was rocked by grenade explosions on two separate days last December, which police said was the work of Somalia’s Al-Qaeda-linked Islamist al Shabaab rebels.

On Tuesday, police with sniffer dogs and bomb disposal officers searched the building after the grenade was found in a store near the prime minister’s office.

No other explosive devices were found, and police said it was too early to say who could have planted the grenade.

“We got a report that a hand grenade had been found near the premises of the prime minister’s office,” Charles Owino, the deputy police spokesman, told reporters.

Odinga’s office is located across a busy street from that of President Mwai Kibaki in a part of the capital dotted by a cluster of government offices, including parliament.

Two people were killed last December when a bag containing a hand grenade exploded as it was about to be carried onto a bus bound for the Ugandan capital Kampala. Earlier that month, three Kenyan policemen were killed in a grenade and gun attack blamed by police on al Shabaab.

Six people were killed in a grenade attack on a political rally at a park in Nairobi in June last year. Analysts attributed the attack to those opposing the adoption of a proposed constitution.