ISTANBUL (AFP) - Turkish police stormed an Istanbul park on Saturday after protesters defied an ultimatum from Prime Minister Recep Tayyip Erdogan to end their occupation of the site, in a major escalation of more than two weeks of anti-government unrest.

Two hours after Mr Erdogan warned of police intervention if protesters did not leave Gezi Park, the epicentre of nationwide protests, officers entered the green patch firing volleys of tear gas and jets of water.

Thousands of campers scrambled as police trampled tents, pulled down banners and broke down barricades in the park. A city cleaning crew then swiftly moved in to clear the site as darkness fell.

Earlier, police also used tear gas and bursts of water to disperse hundreds of demonstrators from Taksim Square, which borders Gezi Park.

The police action sent tensions soaring in the civil unrest in which four people have been killed and nearly 7,500 injured, and has posed the biggest challenge yet to the decade-long rule of Erdogan's Islamic-rooted government.

The trouble first began when a peaceful sit-in to save Gezi Park's 600 trees from being razed in a redevelopment plan prompted a brutal police response on May 31.

The crackdown sparked an outpouring of anger, snowballing into countrywide demonstrations against Mr Erdogan, seen as increasingly authoritarian.

Tensions boiled over on Saturday after demonstrators in Gezi Park refused to clear out in return for Mr Erdogan's promise that he would suspend the site's controversial redevelopment.

Rebuffed, Mr Erdogan issued a stark warning at an election rally attended by tens of thousands of supporters of his ruling Justice and Development Party (AKP) in the capital Ankara.

"We have an Istanbul rally tomorrow (Sunday)," the premier told cheering loyalists in a sea of red Turkish flags.

"Let me put it clearly: empty Taksim. If it is not evacuated, this country's security forces know how to evacuate there," he said.

The United States and other Western allies have widely criticised Mr Erdogan's handling of the crisis, undermining Turkey's image as a model of Islamic democracy.

Mey Elbi, a 39-year-old yoga teacher, was in the park when police entered on Saturday.

"They took our goggles and gas masks," she told AFP, referring to items the protesters carry against tear gas attacks.

"I won't give up," she vowed. "We're angry, this is not over. The world has seen that together, we can stand up to Tayyip." Istanbul governor Huseyin Avni Mutlu said 29 people have been injured but added that none of them were in serious condition.

After days of taking a combative stance against the demonstrators, dismissing them as "looters" and "extremists", Mr Erdogan made an unexpected conciliatory gesture on Friday by holding his first talks with the Taksim Solidarity group, seen as most representative of the protesters.

He agreed to abide by a court-ordered suspension of the park project - a move welcomed by the protesters. He also said that if the court rules that the Gezi Park redevelopment is legal, he wants to hold a popular vote on plans to build a replica of Ottoman-era military barracks on the site.

But in an official response, the protest group said the government had failed to address their list of demands, including a call for arrested demonstrators to be released and for police chiefs in cities that saw violent clashes to be sacked.

"We will continue our resistance in the face of any injustice and unfairness taking place in our country," Taksim Solidarity said in a statement.

"This is only the beginning." Huseyin Celik, deputy AKP leader, said Mr Erdogan felt "cheated" after protesters rejected his olive branch.

Mr Celik added that police had warned protesters "time and again" to leave Gezi Park.

"I hope nobody will be hurt... I hope we will leave this behind as a bad dream or a nightmare," he told local television.

Opponents have accused Erdogan of repressing critics and of forcing conservative Islamic policies on the mainly Muslim but staunchly secular nation - including religious education reforms and restrictions on alcohol sales.

While opposition to him is intense, the 59-year-old, who has been 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 in the country of 76 million people.

Mr Erdogan has repeatedly urged supporters to answer the protesters by voting for his AKP in next year's local polls.

The premier has insisted that this weekend's first election rallies are not meant as a showdown, but he gave his speech in Ankara standing in front of a banner that read: "The game is over. It's time to write history".