German Foreign Minister Guido Westerwelle on Wednesday said images of demonstrators in Istanbul being chased down by riot police were "disturbing" and warned that the Turkish government was sending the "wrong message".

Reacting after Turkish security forces retook control of Taksim Square, the flashpoint of nationwide unrest, Westerwelle urged Ankara to respect the rights of free speech and assembly.

An uneasy calm returned to Istanbul's protest square early Wednesday after running clashes between riot police and protesters, as Turkish leader Recep Tayyip Erdogan vowed zero tolerance for the mass demos.

Hours earlier, Taksim Square had resembled a battle scene, swathed in acrid smoke 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.

Thousands of demonstrators squared off against riot police on Tuesday and defiantly packed an Istanbul square after Turkish Prime Minister Recep Tayyip Erdogan warned he had "no more tolerance" for the mass protests against his Islamic-rooted government.

Hundreds of police stormed Taksim Square, the epicenter of nearly two weeks of unrest, in the early morning, firing tear gas and jets of water. They fought hours-long battles with protesters and used bulldozers to clear the barricades erected by demonstrators after police pulled out of the area on June 1.

The escalation in the clashes between Turkey's government and protesters could hurt one of the world's recent economic success stories, spelling uncertainty for a country that has become a source of growth and stability in a region hit by recession and unrest.

At first, the 12-day protests, which were sparked by plans to bulldoze and redevelop a park in central Istanbul but have widened to incorporate concerns over the government of Prime Minister Recep Tayyip Erdogan, were largely peaceful and limited in size and scope. They weren't very different from the Occupy Wall Street protests in New York.

A key suspect in twin car bombings that killed 52 people in a Turkish city near the Syrian border last month has been arrested while trying to flee to country, local officials said Tuesday.

"An individual who appears to be one of the main suspects of the twin attacks was arrested on the evening of June 10 as he tried to cross the (Syrian) border," the governor's office in the southeastern province of Hatay, where the bombs went off, said in a statement.

Riot police re-entered Istanbul's protest square Tuesday, sending demonstrators scrambling with tear gas and water cannon in a fresh escalation of unrest after Turkish Prime Minister Recep Tayyip Erdogan said he would meet with protest leaders.

Hundreds of police officers backed by armored cars moved in on Taksim Square in the early morning in their first return to the site after pulling out on June 1, saying they wanted to remove makeshift barricades erected by protesters and clear the area of flags and banners.

Turkish Prime Minister Recep Tayyip Erdogan on Wednesday will meet with protest leaders whose mass anti-government demonstrations, now in their 11th day, have rocked the country, his deputy said.

"Our prime minister has given an appointment to some of the groups leading these protests.... They will be briefed on the facts and our prime minister will listen to their thoughts," deputy premier Bulent Arinc said Monday, as fresh protests raged in Istanbul, Ankara and other cities.

With few customers to serve, the moneychangers near Istanbul's Taksim Square jabber in annoyance behind their glass partition.

Their volume of business has dropped 70 percent since mass protests broke out against Prime Minister Recep Tayyip Erdogan over a week ago.

Turkey's Prime Minister Recep Tayyip Erdogan declared on Sunday that the patience of his Islamic-rooted government "has a limit" as he went on the offensive against mass protests to his decade-long rule.

"We remained patient, we are still patient but there's a limit to our patience," Erdogan told thousands of cheering supporters of his Justice and Development Party (AKP) as he landed at Ankara's airport, hours after rallying loyalists in two other Turkish cities.

The pro-secular protests rocking Turkish cities have sent ripples across the Arab world, unnerving Islamist leaders who have long touted Turkey as a successful model of political Islam, analysts say.

Thousands of Turks have joined in mass anti-government demonstrations, defying Prime Minister Recep Tayyip Erdogan's call to end the worst civil unrest of his decade-long rule.