Turkish riot police have entered Istanbul's Taksim Square, where anti-government protesters have been staging demonstrations for close to two weeks, according to BBC.
Hundreds of officers have been using tear gas and water cannon to disperse activists.
The move comes after Prime Minister Recep Tayyip Erdogan agreed to meet the protest organisers on Wednesday.
The unrest was sparked by a police crackdown on a local protest over an Istanbul park.
The protests then widened, with demonstrators accusing Mr Erdogan's government of becoming increasingly authoritarian and trying to impose conservative Islamic values on a secular state.
Backed by armoured vehicles, police wearing helmets and carrying shields gathered around the square early on Tuesday.
They then began to move past barricades erected by protesters.
The BBC's Mark Lowen, who is in the square, says it is a deliberate show of force.
The demonstrations are now in their 12th day, with activists controlling much of Taksim Square.
The unrest was sparked after police moved to suppress environmental protests over the redevelopment of Gezi Park, which is part of Taksim Square, on 31 May.
More than 5,000 people have been injured and three people have died since the protests began.