Kenyan police have used water cannon and tear gas to disperse protesters, as a controversial draft constitution was presented to parliament for debate.
Police banned the protests in case of "chaos and mayhem"
Five people were arrested, as hundreds marched in central Nairobi.
President Mwai Kibaki was elected in 2002, partly on a promise to reduce the president's powers within 100 days.
Almost three years later the constitution has not been changed and critics accuse Mr Kibaki of being unwilling to give up his powers.
Police were out in force from early morning after banning the protests, saying troublemakers were planning "chaos and mayhem".
"Parliament keep off! It is our Constitution," read banners carried by protesters.
They scuffled with police at various points around parliament.
Critics say President Kibaki is unwilling to relinquish powers
"There is no jail to accommodate the people of Kenya," one protester told the BBC.
Under the latest draft constitution, a new post of prime minister is being created, however Mr Kibaki's critics say the president retains most powers.
The issue has deeply divided the Narc coalition which united behind Mr Kibaki in the 2002 election.
The Liberal Democratic Party, led by Raila Odinga, who had been promised the post of prime minister, opposes the changes, as has the opposition Kanu party.
Kanu and LDP MPs shouted "shame, shame" as the changes were presented to parliament.
"Judging by the number of policemen, horses and dogs, one would think Kenya is a police state or in a state of emergency," William Ruto, an MP and Kanu secretary-general, told Reuters news agency.
Internal Security Minister John Michuki said the government was obliged to control the protesters.
"There was a threat to law and order. We have to keep peace," he told parliament.
MPs are due to start debating the constitution in earnest on Wednesday and the protesters have vowed to return.
If parliament adopts the constitution, it is due to be put to a referendum in October.