Kenyan riot police armed with teargas, batons and a water cannon vehicle fought running battles on Saturday with hundreds of people defying a ban on a rally called to press for constitutional reform.
A Reuters correspondent saw paramilitary police in camouflage gear and baton-wielding riot police, some on horseback, forcing back demonstrators from the edge of Uhuru Park close to Nairobi's central business district.
The government said on Friday it was banning the rally, due to be held in the park, because it feared violence.
A police water cannon vehicle drove up and down the street, repeatedly dousing the crowd with water as it tried to regroup.
A few hundred metres away, dozens of students from Nairobi University threw stones at passing vehicles and police, who charged them and fired teargas canisters.
About 100 youths singing "(President Mwai) Kibaki has let us down" set fire to a tree along the city's main highway.
The Liberal Democratic Party (LDP), a member of the ruling coalition, and the Katiba (Constitution) Watch lobby group had called the rally to demand that the government enact a long-delayed new constitution.
They have accused President Mwai Kibaki of deliberately blocking the constitution, which would considerably trim his powers.
A police helicopter clattered above, partly drowning out the chants and blaring car horns. Most shops in the city remained closed.
There were no immediate reports of injuries.
Kenya has been experiencing its worst political crisis since Kibaki won December 2002 elections, marked by rows over power-sharing proposals in the draft constitution which threaten to rip apart the shaky ruling National Rainbow Coalition (NARC).
Kibaki had promised to have the constitution in place by Wednesday, and the missed deadline has angered the LDP and Katiba Watch.
"Kibaki has taken Kenya many steps backwards in terms of democracy. We are going to plan our next move," Martin Shikuku, an official of the lobby group, told reporters at a city hotel earlier on Saturday.