ISTANBUL (AP) - Violence has flared in Istanbul between a group of demonstrators and police on the fourth day of protests set off by a police crackdown of a peaceful environmental protest.

The private Dogan news agency said police fired tear gas at the group in an area close to Prime Minister Recep Tayyip Erdogan's Istanbul offices on Monday.

The protesters responded by hurling stones.

The agency said as many as 500 protesters were detained overnight on Monday after police broke up a protests by several thousands of people in the capital Ankara.

Turkey's Fox television reported 300 others detained in a similar crackdown in Izmir, Turkey's third largest city.

The demonstrations that grew out of anger over excessive police force have spiraled into Turkey's biggest anti-government demonstrations in years, challenging Mr Erdogan's power.