The United Nations called for a full investigation into allegations of excessive use of police force against anti-government demonstrators while Turkey's main union federation launched a two-day strike over what it branded "state terror".

The embattled government of Prime Minister Recep Tayyip Erdogan sought to ease tensions by apologizing to wounded demonstrators and admitting that actions by security forces against people with "rightful demands" had caused the situation to get out of hand.

"The government has learnt its lesson from what happened," Deputy Prime Minister Bulent Arinc said at a press conference. "We do not have the right and cannot afford to ignore people. Democracies cannot exist without opposition."

Arinc called on "responsible citizens" to stop the protests which he said had left about 300 people wounded in five days although rights groups have put the number of injured much higher and two people have lost their lives.

Violence raged overnight as riot police in Istanbul and Ankara fired tear gas and water cannon at flag-waving protesters who set cars ablaze, hurled stones and bellowed angry slogans against Erdogan and his policies, chanting "Tayyip, Resign!"

The situation was calm by early afternoon, with a few scattered groups of demonstrators waving the flags of unions and political groups in the heart of Istanbul, one of the world's most visited cities.

Erdogan has remained defiant, lashing out at "extremists" and dismissing accusations he is a "dictator" as he pressed ahead with a tour of North Africa despite the trouble at home.

The violence first erupted after police cracked down on a peaceful rally in Istanbul against plans to build over Gezi Park, a rare green spot adjoining the city's main Taksim Square. But it quickly mushroomed into broader protests in dozens of other cities by Turks who accuse the government of pushing conservative Islamic reforms.