Opposition groups opposed to President Mwai Kibaki's handling of the constitutional reform process have called three days of protests before Friday's deadline for parliament to finalise its version of the new treaty prior to a referendum.

"This is just a beginning. We are going to paralyse this city," one protest leader Otieno Ombok, of the Katiba (Constitution) Watch group, said in Nairobi.

Authorities have outlawed the demonstrations.

Large numbers of riot police with shields and batons, some on horseback and others waiting in trucks, scuffled with and blocked the demonstrators at several points around parliament.

Mr Kibaki's government is accused of watering down initial cross-party recommendations for rewriting the east African nation's constitution, drawn up by Kenyans during the Lancaster House negotiations before independence from Britain in 1963.

The main bone of contention is the powers of the presidency, which the latest draft from a government-dominated parliamentary committee leaves virtually intact, in spite of a recommendation that most authority go to a powerful new post of prime minister.

Opposition groups are angry that parliament has redrawn last year's so-called Bomas draft of the constitution which included wide consultation with grassroots groups.

Former President Daniel arap Moi's 24-year rule was marked by violent repression of protests, especially in the 1990s.

Mr Kibaki came to power in 2002 promising greater respect for democracy and rights, but last year faced days of violent protest over the constitution during which one person died.

Today, several activists stood in front of police lines, even when hosed down by water cannon.

Before that, Reuters reporters saw four people arrested and bundled into an unmarked police car.

"Why am I under arrest? Take this petition. We're not in a police state. We're not thieves," one of the detainees Koitamet Ole-Kina shouted to reporters.

The protesters carried a banner declaring: "Parliament, keep off! It's our constitution!" as they danced and sang slogans.

After initial support, opposition parties - and one of the ruling coalition groups parties, the Liberal Democratic Party - boycotted the constitutional review process several months ago.

Senior members of Kibaki's National Rainbow Coalition (NARC) said opposition parties were stirring up trouble hypocritically after initially backing the constitution review.

There were angry clashes in parliament, where proper debate on the constitution was due to start tomorrow.

"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 secretary-general of the opposition Kenya African National Union (KANU), said.

Internal Security Minister John Michuki said the government was obliged to control the protesters.