Riot police in the Brazilian city of Sao Paulo have clashed with other police officers who are on strike.
The confrontation happened when the striking officers tried to force their way past barriers to reach a building used by state governor Jose Serra.
Tear gas and rubber bullets were used to keep back the demonstrators and 15 to 20 injuries have been reported.
Mr Serra later said many protesters had not been police, but members of other unions and political groups.
The clashes happened in the Morumbi district of Sao Paulo as the police officers, who have been on strike for a month, tried to reach the building used by the governor.
The strikers wanted to meet him in order to negotiate a 15% pay rise.
In a statement, the authorities said the trouble began when strike leaders accepted a proposal to receive an official representative in the area where they were staging the protest.
Live television picture showed pushing and shoving and some punches being thrown as the protesters pushed forward. Riot police dressed in black then began firing tear gas, stun grenades and rubber bullets.
In one incident, a marked police vehicle could be seen trying to force its way through a police cordon, and there were a number of heated exchanges.
Police officers on horseback were also brought in to try to restore calm.
Hospitals nearby received a small number of people with slight injuries, including a senior officer who had been involved in the negotiations with the protesters.
The striking officers are from the branch of the state police responsible for detective work and criminal investigations, while those trying to hold the demonstrators back are from the military police, whose role is broadly to maintain law and order on the streets.
However, Governor Serra has claimed many of those taking part in the protest were politically motivated and were not in fact police officers.
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