ANKARA, Turkey -- As riot police used tear gas against protesters for a fourth straight day in Istanbul, Turkey's president and prime minister displayed wide differences Monday in their responses to those taking to the streets.

Turkey has seen violent demonstrations since Friday, when police launched a dawn raid against a peaceful sit-in protesting plans to cut down trees in Istanbul's main Taksim Square. Since then, the demonstrations by mostly secular-minded Turks have spiraled into Turkey's biggest anti-government disturbances in years, challenging Prime Minister Recep Tayyip Erdogan's power.

A Turkish doctors' group said Monday that one protester died of injuries after a vehicle slammed into crowd.

The protests are seen as a display of frustration against Erdogan, who has appeared to be increasingly authoritarian and is accused of forcing his conservative, religious Islamic outlook on the lives of secular Turks.

Erdogan, who has been in power since 2003 after winning three landslide elections, inflamed tensions by calling protesters "a bunch of looters" and by branding them a "minority" who are trying to force demands on his majority.

In contrast, President Abdullah Gul took a more conciliatory line, celebrating peaceful protest as a democratic right.

The two men are expected to compete against each other next year in Turkey's presidential election.

On Monday, Erdogan again dismissed the street protests as being organized by extremists and angrily rejected comparisons with the Arab Spring uprisings.

"We already have a spring in Turkey," he said, alluding to the nation's free elections. "But there are those who want to turn this spring into winter.

"Be calm, these will all pass," he said.

Turkey's main stock exchange dropped 6.4 per cent upon opening Monday, as investors worried about the destabilizing effect of the demonstrations on the economy.

Erdogan played down its significance, saying: "It's the stock market, it goes down and it goes up. It can't always be stable."

Appearing defensive and angry, he lashed out at reporters who asked whether the government had understood the message by protesters.

"What is the message? I want to hear it from you," Erdogan retorted. "What can a softened tone be like? Can you tell me?"

He spoke to reporters before leaving on a four-day trip to Morocco, Algeria and Tunisia.

Gul said democracy is not just going to the ballot box.

"When we speak of democracy, of course the will of the people is above all," Gul said. "But democracy does not mean elections alone. There can be nothing more natural for the expression of various views, various situations and objections through a variety of ways, besides elections."

He added: "The views that are well intentioned have been read, seen and noted and the messages have been received."

Some of the protesters clashed with police, but most demonstrated peacefully, chanting calls for Erdogan to resign. Those who did not take to the streets banged on pots and pans from windows.

There was scattered violence in areas close to Erdogan's offices in Istanbul and in Ankara. The Dogan news agency said police fired tear gas at one close to Erdogan's Istanbul office. The protesters responded by hurling stones.

The agency said as many as 500 people were detained overnight Monday after police clashed with more militant protesters and then moved in to break up several thousand people demonstrating peacefully. Turkey's Fox television reported 300 others were detained in a similar crackdown in Izmir, Turkey's third-largest city.

Social media were awash with reports and videos of police abuse. Authorities have said police excesses would be investigated, but they appeared to continue unabated.

Fox showed footage of police telling one group sheltering by the side of a building to come out, reassuring them that nothing would happen, then shooting a gas canister at one of them.

Another group of protesters took control of a large bulldozer in Istanbul and drove it toward police water cannon, Dogan news agency footage showed. Medics were seen tending to people injured in the skirmishes or affected by gas at a mosque close to the palace.

Erdogan described some of the protesters as "naive, decent and participating (in demonstrations) by following information on social media" but claimed the protests were being organized by Turkey's opposition party and extremist groups.

He also blamed the protest on "internal and external" groups bent on harming Turkey, said the country's intelligence service was working on identifying them and threatened to hit back at them.

"We shall be discussing these with them and will be following up, in fact we will also settle accounts with them," he said.

In neighbouring Iraq, Prime Minister Nouri al-Maliki said on his official website that his government was worried about the security implications of the situation in Turkey, saying the country was "an essential part of the stability of the region."

"We believe that resorting to violence will widen the circle (of violence) ... in the region, and we call for restraint," he said.

Iraq and Turkey share a long, mountainous border. Iraq is home to an ethnic Turkomen minority, centred around the disputed Iraqi city of Kirkuk.

The two countries' relationship has been increasingly strained over growing Turkish ties to Iraq's largely autonomous northern Kurdish region, and over Turkey's support for the Sunni rebels fighting to topple the Syrian regime.

The two-year Syrian civil war, which has already killed 70,000 people, is exacerbating sectarian divisions within Iraq. Baghdad has warned that the fall of the Iranian-backed Syrian government could ignite a wider conflict in the region.

Associated Press writer Adam Schreck in Baghdad contributed to this report.