Riot police stormed an Istanbul protest square Tuesday, firing volleys of tear gas at firework-hurling demonstrators after Turkish leader Recep Tayyip Erdogan warned he had "no more tolerance" for the mass demos against his Islamic-rooted government.

Taksim Square turned into a battle scene as darkness fell, clouds of acrid smoke filling the site as police dispersed tens of thousands of protesters chanting "Erdogan, resign!" and "Resistance!", on the worst night of violence in 12 days of nationwide unrest.

After riot police sent the large crowd scrambling with bursts of gas and jets of water, cat-and-mouse games with smaller groups of demonstrators continued into the night.

Fresh unrest also erupted in the capital Ankara, which has seen several bouts of violence in recent days, with police using gas, pepper spray and water cannon on some 5,000 protesters near the US embassy. Some threw rocks in response.

The unexpected police intervention in Istanbul's Taksim Square, which began early Tuesday morning, marked the first time officers returned to the area since pulling out more than a week ago. They fought hours-long battles with clusters of demonstrators, some of whom hurled back fireworks and Molotov cocktails.

The police also brought in bulldozers to clear barricades erected by demonstrators.

The assault on Taksim Square surprised protesters, many of whom were dozing in nearby Gezi Park, because it came after Prime Minister Erdogan said he would meet with protest leaders on Wednesday, his first major concession since the trouble began.

But the premier made no mention of the olive branch Tuesday and resumed his tough stance against the demonstrators who have put up the biggest challenge yet to his decade-long rule.

"This episode is now over. We won't show any more tolerance," the premier told cheering lawmakers of his Justice and Development Party (AKP) in a speech broadcast live on television.

"Can you believe that? They attack Taksim, gas us in the morning just after proposing talks with us?" said 23-year-old Yilmaz.

"We won't abandon Gezi," he vowed. "I am not afraid of their water cannon, it'll be my first shower in three days."

The nationwide unrest first erupted after police cracked down heavily on May 31 on a campaign to save Gezi Park from redevelopment.

The trouble spiralled into mass displays of anger against Erdogan, who is seen as increasingly authoritarian, tarnishing Turkey's image as a model of Islamic democracy.

Erdogan said on Tuesday that four people, including a policeman, had died in the unrest. Nearly 5,000 demonstrators, many of whom are young and middle-class, have been injured.

-- Taksim 'tarnishing Turkey image' --

In a rousing speech to lawmakers, Erdogan urged "sincere" protesters in Gezi Park to pull back, warning that their environmental campaign was being hijacked by "an illegal uprising against the rule of democracy".

In a symbolic gesture, police removed anti-Erdogan banners from a building overlooking Taksim and replaced them with a single Turkish flag and a large portrait of Mustafa Kemal Ataturk, the father of modern Turkey, whose image has also been adopted by the protesters.

Istanbul governor Huseyin Avni Mutlu sought to justify the intervention, saying the protesters' takeover of Taksim Square "tarnished the country's image before the eyes of the world", and assured demonstrators police would not storm Gezi Park.

Inside the park, filled with smoke wafting in from Taksim Square, volunteers offered first aid to injured demonstrators.

One female volunteer, who did not wish to be named, said 25 people had been transferred to hospital. Many of the injured suffered burns from tear gas canisters, she said.

Amnesty International and Human Rights Watch (HRW) condemned the police crackdown and called for dialogue.

"Teargassing tens of thousands of protesters in Taksim Square won't end this crisis," HRW said. "The police brutality has to stop."

Opponents accuse Erdogan of curbing freedoms and of pushing conservative Islamic values on the mainly Muslim but constitutionally secular nation.

But the 59-year-old, in power since 2002, remains the country's most popular politician. His AKP has won three elections in a row and took nearly half the vote in 2011, having presided over strong economic growth.

Erdogan has urged loyalists to respond to the demonstrators by voting for the AKP in local polls next year.

The first campaign rallies will be staged in Ankara and Istanbul this weekend and are expected to gather tens of thousands of party faithful.

Turkey, a country of 76 million at the crossroads of East and West, is a key strategic partner in the region for the United States and other Western allies. Many of them have criticised Erdogan's handling of the crisis.