Last Updated: Saturday, 28 February, 2004, 14:46 GMT
About 200 people took part in what the police described as an illegal protest outside police headquarters.
Carrying posters reading "Beware of Violent Police", the demonstrators demanded action against corruption and an enquiry into the death of detainees.
At least 17 people, including politicians, were arrested.
There is considerable public concern in Malaysia at the high number of deaths in police custody and fatal shootings by officers.
In December, Prime Minister Abdullah Badawi announced the setting up of a wide-ranging royal commission that will reassess the role of the force, its operating procedures, recruitment and training practices.
Kuala Lumpur police chief Mustafa Abdullah told reporters they turned water cannon on the crowd after they refused police warnings to disperse.
Police were only expecting a memorandum to be handed over, but some of the activists sat down by the road while others carried posters highlighting alleged cases of abuse, Mr Mustafa said.
The organisers accused police of being heavy-handed.
"It was totally unexpected and unprovoked. It was just a small gathering but police have acted with such unnecessary force. It's very unprofessional," National Justice Party vice-president Tian Chua, who was among those detained, told AFP.
But the police chief defended their actions.
The protesters should have referred their complaint to the newly-formed commission, rather than taking to the streets, Mr Mustafa said.
The commission's primary aim will be to end brutality and graft within the ranks, while increasing professionalism and instilling a greater respect for human rights.