Tens of thousands of people have rallied in Bangkok, calling for the resignation of Prime Minister Yingluck Shinawatra. The main opposition plans to boycott upcoming snap elections.
Protesters brought parts of Bangkok to a standstill on Sunday, blocking key intersections on the road network.
Thailand's National Security Council chief Paradorn Pattanatabut told the AFP news agency that a minimum of 150,000 people were involved, although protesters put the figures far higher.
The protest movement's lead figure, former secretary general of the main opposition Democrat Party Suthep Thaugsuban, had said prior to the demonstrations that he was hoping to mobilize one million.
Addressing a large crowd at the city's Democracy Monument, Suthep encouraged protesters to disrupt the snap elections called for February 2 by blockading the Election Commission, where the registration process is scheduled to start on Monday.
"Whoever wants to go inside to register will have to pass through us," Suthep said. "If we do not hold the country by February 2, we will shut the country down. No one will go to vote."
The Democrat Party, which Suthep left to lead the protest movement, said on Saturday that it will boycott the February elections, without advocating efforts to disrupt the vote. The opposition has instead called for the establishment of a loosely-defined "people's council," charged with implementing reforms on an interim basis, ahead of a fresh vote.
Opinion polls were pointing to defeat for the Democrat Party prior to the confirmation that it would not compete.
'What should we hold on to?'
Prime Minister Yingluck Shinawatra called the snap elections on December 9, in response to a month of public protests triggered by proposed legislation that might have provided her brother, former Prime Minister Thaksin Shinatwatra, with an amnesty from corruption convictions.
The protesters allege, among other things, that Yingluck's government and Pheu Thai Party remains heavily influenced by Thaksin, currently living in self-imposed exile.
Yingluck on Sunday said that the elections were necessary to safeguard Thailand's democracy.
"If we don't hold on to the democratic system, what should we hold on to?" the prime minister asked reporters. "If you don't accept this government, please accept the system."
Thaksin's government was removed from power in a 2006 coup, following another Democrat Party boycott of elections earlier that year, but the country's military has so far resisted some opposition calls for renewed intervention.
The Pheu Thai Party won just under 50 percent of the vote and a narrow majority in terms of parliamentary seats in the country's last general election in 2011.
The vote was clearly divided along geographical lines, the opposition Democrat Party claimed almost all of its seats along Thailand's western border and in the south of the country. The Pheu Thai Party was strongest in the country's poorer, rural northern and eastern regions.
msh/jm (AFP, AP, dpa)