Crowds of Thai nationalists, known as “yellow-shirts”, have protested in Bangkok over the government’s handling of a border dispute with Cambodia.
Some 2,000 members of the People’s Alliance for Democracy (PAD) set up camp near Government House.
They are calling on Prime Minister Abhisit Vejjajiva to revoke a pact with Cambodia on settling a land row.
The debate has been fuelled by the arrest in Cambodia in December of seven Thais accused of illegal entry.
A yellow-shirt activist remains in jail on espionage charges.
The PAD is the group that shut down Bangkok’s airports in 2008 in a bid to force the previous government of allies of ousted leader Thaksin Shinawatra to step down.
It has made nationalism, and in particular border disputes with Cambodia, a key part of its political platform.
The protesters’ demands have been rejected by Mr Abhisit, who came to power after the airport protests, but relations have since soured.
“We did not come here on a whim. We have waited for the prime minister to deal with this for two years and the situation is only getting worse, like every other issue,” Chamlong Srimuang, a PAD leader, was quoted by Reuters as saying.
“This government is proving to be a big disappointment,” he said.
The border issue has been a long-standing flashpoint for the two neighbours.
It was reignited in 2008 when Cambodia applied to Unesco to have a hill-top temple – to which both countries lay claim – declared a World Heritage Site.
The application was backed by the government in power in Bangkok at the time – but was opposed by those who said the move threatened Thailand’s sovereignty.
Since then there have been sporadic exchanges of fire in areas around the disputed Preah Vihear temple.
Analysts say the yellow-shirt demonstration appears mainly intended to pressure the government ahead of elections that must be called this year.
The protest comes two days after about 30,000 rival “red-shirts” held their own protest in Bangkok.
In April and May last year, more than 90 people died in clashes between red-shirts and government troops in Bangkok, during protests demanding snap elections.