Updated

Riot police in Istanbul fired water cannon and tear gas Monday to disperse pockets of protesters on the sidelines of a demonstration called by labor groups who hope to capitalize 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 labor 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 Monday, Deputy Prime Minister Bulent Arinc floated the prospect that authorities could still call in the military.

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

Monday's labor-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 feet 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 labor 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.

Strikes, however, often have little visible impact on daily life in Turkey, a country of about 75 million, and the call to walk off the job Monday had limited fallout beyond the demonstrations.

Unionists in Istanbul hoped to reach Taksim Square on Monday afternoon. But police have maintained a lockdown on the square after unrest continued in pockets of the country overnight.

The standoff between police and protesters began as an environmentalists' rally over the government's plans to tear down trees and redevelop Gezi Park. But a police crackdown on May 31 lit a fuse on much broader anger and morphed the movement into a protest against Erdogan's government.

His opponents have grown increasingly suspicious about what they consider a gradual erosion of freedoms and secular Turkish values under his Islamic-rooted party's government. It has passed new curbs on alcohol and tried, but later abandoned its plans, to limit women's access to abortion.

Five people, including a policeman, have died and more than 5,000 have been injured, according to a Turkish rights group.

Erdogan has been praised for shepherding Turkey to strong economic growth as many other world economies lagged. But his government's handling of the protests has dented his international reputation.

He has blamed the protests on a nebulous plot to destabilize his government and repeatedly lashed out at reports in foreign media and chatter in social media about the situation.

In a speech Monday, he again took aim at the European Parliament for its criticism of his management of the crisis, saying it was "siding with people who are attacking people's freedom" -- a reference to the protesters. He said they were "anti-democratic" because they had occupied Gezi Park, and accused them of harming private and public property.

Turkey has had grand ambitions on the international stage, including a bid to host the 2020 Olympics and its long-standing aim to join the European Union. But criticism of Erdogan's response to the crisis may have damaged such goals.

Chancellor Angela Merkel of Germany, which is home to some 3 million Turks, told German broadcaster RTL she was "appalled" to see footage of police forces moving in to clear Istanbul's Gezi Park over the weekend. She criticized the crackdown by Turkish police as "much too strong."

"What is happening at the moment in Turkey does not correspond to our idea of the freedom to protest and the freedom of speech," she said. Germany is Europe's biggest economy and a key player in EU decision-making.

The labor walkout was the second since the protests began. Another took place June 5.

"The first one, we said it's a warning for the government, to listen the streets, to listen the message from the demonstrators, and we asked them to stop this police violence," said Kivanc Eli Acik, a labor leader.

"But after that day, rather than stopping the violence, the excessive police violence and intervention is going much, much bigger. So this is the second warning, the second strong message to the government," he added.