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Thai police abandoned their defense of the besieged government headquarters on Tuesday, allowing unruly protesters to cross barricades in a dramatic move that eased tensions ahead of the revered king's birthday.

Several days of street battles between demonstrators and security forces suddenly gave way to hugs and smiles after police said they would no longer use force against protesters trying to storm Prime Minister Yingluck Shinawatra's offices, as well as their own headquarters.

The sudden change in tactics came as the nation prepares on Thursday to celebrate King Bhumibol Adulyadej's 86th birthday, a day normally marked with  calm and reverence for the monarch.

"There's a mutual understanding that everything must be calm and orderly on this auspicious day," National Security Council chief Paradorn Pattanatabut told AFP.

"It's a positive sign and talks could be held after the king's birthday. It will take time to solve the problem," he added.

The protests, aimed at unseating Yingluck's government and replacing it with an unelected "people's council," are the latest bout of unrest in the kingdom since royalist generals ousted the premier's brother Thaksin Shinawatra in a coup seven years ago.

Protest leader Suthep Thaugsuban, a former deputy premier, said the fight to unseat the government was not over.

"It's a partial victory but it's not final because the Thaksin regime is still in place. You cannot go back home yet. We have to continue our struggle," he told his supporters.

Thailand's long-running political conflict broadly pits a Bangkok-based elite backed by the military and the palace against rural and working-class voters loyal to Thaksin, a billionaire businessman turned populist politician.

In the latest twist, demonstrators were unexpectedly allowed to amass in the grounds of Government House without resistance.