Helmeted police officers used truncheons to beat protesters who hurled stones at them outside government buildings in Nairobi on Wednesday.
Witnesses said one man was seriously injured in the clashes with nearly 200 demonstrators.
Standing in front of police lines, scores of people sang songs calling for an uprising.
"Even if you kill us, we still want our constitution," they chanted.
Groups opposed to President Mwai Kibaki's handling of the constitutional reform process have called for three days of protests before Friday's deadline for parliament to finalise its version of the new treaty prior to a referendum.
The protests have been banned by the authorities.
The most contentious issue is the powers of the president, which the latest draft from a government-dominated parliamentary committee leaves virtually untouched, despite a recommendation from a cross-section of Kenyans, including politicians and religious leaders, that most authority go to a new prime minister post.
Critics say cronyism is rife in
President Mwai Kibaki's cabinet
Most Kenyans are disillusioned with Kibaki's two-and-a-half years in office, saying he has failed to live up to his 2002 election pledge to end the tribal politics and corruption that flourished under predecessor Daniel Arap Moi's 24-year rule.
Critics say cronyism is rife in Kibaki's government, which they accuse of watering down initial cross-party recommendations for rewriting Kenya's constitution, drawn up by Kenyans before independence from Britain in 1963.
"We have seen the government is coming to kill people, but they should know they cannot force a constitution down the people's throats," Koitamet Ole-Kina, of the Katiba (Constitution) Watch group, said.
Members of Kibaki's National Rainbow Coalition have accused opposition parties of hypocrisy, saying they were stirring up trouble after initially backing the constitutional review.
Kenya's main daily newspapers urged for calm in the streets and compromise between the political parties representing the east African nation of 32 million.
"It would be a sad mark in this country's history if the constitution was lost to politics and Kenyans ended up with a flawed document," the Daily Nation said in an editorial.