Riot police in Istanbul fire water cannon and tear gas at protestors

ISTANBUL (AP) - Riot police in Istanbul fired water cannon and tear gas on Monday to disperse pockets of protesters on the sidelines of a demonstration called by labour groups who hope to capitalise on weeks of initially small-scale activism to register broader discontent.

The demonstrations were the latest challenge to the Islamic-rooted, conservative government of Prime Minister Recep Tayyip Erdogan, who has drawn scorn abroad for his tactics against peaceful activists in two weeks of protests and exposed fault lines in Turkey's democracy.

Two major labour groups urged their members to hold the one-day strike and participate in demonstrations in response to a police crackdown against activists who led a wave of protests that have centered on Istanbul's Taksim Square and nearby Gezi Park in recent weeks.

A rally in Ankara took place peacefully, and there were no immediate signs that the police operation in Istanbul had provoked major clashes in the afternoon. Earlier, Turkey's interior minister warned that anyone joining unlawful demonstrations would "bear the legal consequences."

Meanwhile, in a sign of tensions between rival groups, images from Dogan news agency showed crowds of government supporters facing down some protesters. Some chanted "the hands targeting the police should be broken."

The government expressed increasing exasperation over more than two weeks of street demonstrations, including a sit-in in Gezi Park, overwhelmingly peaceful demonstrations and - at times - clashes between stone-throwing youths and riot police.

Earlier on Monday, Deputy Prime Minister Bulent Arinc floated the prospect that authorities could still call in the military.

In a TV interview, Mr Arinc stopped short of saying troops would be called in or a state of emergency declared, according to the state news agency, Anadolu. But he said that if the police operations weren't enough to calm the situation, local governors "can benefit from Turkey's military forces" under the law, he said.

The labour demonstrations follow a weekend in which police purged activists from an 18-day sit-in at the park that has come to symbolise defiance against the government, while Mr Ergodan's conservative political base held huge rallies in both Istanbul and Ankara.

Monday's labour-led demonstrations had a more structured feel compared to the counterculture-style sit-in at Gezi and the spontaneous protests of recent weeks. Middle-aged men banged drums and chanting women sat on the ground, hands clasped, as part of the demonstrations.

Earlier, in Ankara, thousands of demonstrators waving union flags, jumping and whistling converged at central Kizilay Square in an uneasy face-off about 50 metres away from riot police and a line of trucks.

Turkey's NTV television reported that riot police issued warnings to the demonstrators to disperse, saying the rally was unlawful and authorities would take action if they did not. After about three hours, the protesters left peacefully.

TV images showed hundreds marching in the Aegean Sea coastal city of Izmir.

Behind the strikes were the KESK confederation of public sector workers and DISK, a confederation of labour unions from industries including transport, construction, health care and media. Together they say they represent 330,000 workers. Small unions that group professionals such as dentists, doctors and engineers also joined in.