Erdogan acknowledged that some Turks have been involved in the protests out of environmental concerns, but insisted that terrorist groups are involved in the unrest in a country seen as a democratic model for the Middle East.

It appeared unlikely that Erdogan's comments would calm demonstrators. Protesters were gathering at an airport in Istanbul in anticipation of the prime minister's return home Thursday night.

Erdogan frequently spends weekends in Istanbul, rather than the capital Ankara. It was unclear whether he chose to land in Istanbul because he has a strong support base going back to his days as the city's mayor.

In Tunisia, Erdogan said he had "love and respect" for protesters with environmental concerns.

"But as I told you earlier, some terrorist groups are involved," he said, claiming that an outlawed left-wing militant group that carried out a suicide bomb attack on the US Embassy in Ankara in February was also involved in the protests.

"They are involved. They have been caught in the streets and on social media," he said.

Erdogan said the Islamic-rooted government had already apologised for the violent police crackdown on the Taksim sit-in, but insisted that tear gas was used everywhere in the world to break up protests.

"Demands cannot be made through illegal means," he said. Erdogan has insisted that democracy happens only at the ballot box, dismissing the demonstrators as an extremist fringe.

Erdogan indicated that the government would go ahead with the redevelopment plans in central Istanbul, insisting the plan would beautify the city, and would include the planting of trees and the construction of a modern theater and opera. Erdogan has earlier said the plans included the construction of a shopping mall.

Some protesters in Taksim Square were skeptical of Erdogan's pledges.

"I do not believe his sincerity," said protester Hazer Berk Buyukturca.