The move followed clashes over the weekend and on Monday, with tear gas and rubber bullets used.

The mood in the capital Bangkok appeared calmer after the police move, which officials said was aimed at avoiding further confrontation.

The Thai prime minister has rejected protesters' demands that she step down.

Yingluck Shinawatra said she was open to negotiations but that calls for the government to be replaced by an appointed council were illegal and unconstitutional.

The protests, which began on 24 November, had been largely peaceful until Saturday, when they became violent.

Over the weekend and on Monday demonstrators tried to break apart police barricades and storm the prime minister's office, Government House, with police using tear gas and water cannon to repel them.
Protest leader Suthep Thaugsuban said on Monday that the protesters would "take over the Metropolitan Police Bureau and make it the people's".

On Tuesday morning - in a surprise and possibly shrewd move - the police took down barriers and razor wire outside their building and it was announced that the protesters were welcome inside, the BBC's Jonah Fisher reports from Bangkok.

Protesters are also being allowed through the barricades outside Government House, the scene of violence on Monday.

The move is seen as an attempt to prevent further clashes.

"In every area where there has been confrontation, we have now ordered all police to withdraw. It is government policy to avoid confrontation," police chief Kamronvit Thoopkrachang told Reuters.

"Today, we won't use tear gas, no confrontation, we will let them in if they want," he said.

With the obstacles removed the demonstrators have so far declined to enter any of the buildings, our correspondent says.

Protesters have described the latest developments as a victory for the people.