BANGKOK - Tens of thousands of anti-government demonstrators massed in Bangkok on Sunday, with a major rival rally due later as Thailand faces its most significant political street action since bloody protests in 2010.
Authorities expect at least 50,000 anti-government protesters to gather by Sunday afternoon, with thousands more "Red Shirts" set to mobilise in another area of the city on Sunday evening in support of prime minister Yingluck Shinawatra's crisis-hit administration.
Both groups have vowed to remain in the capital overnight.
Thailand has been rocked by periodic outbreaks of unrest since divisive former premier Thaksin Shinawatra - Yingluck's brother - was deposed in a military coup seven years ago.
The Thai capital has already faced weeks of opposition-backed rallies sparked by an amnesty bill that could have allowed the return of Thaksin from self-imposed exile - and pardoned those responsible for a deadly military crackdown on his Red Shirt supporters.
The bill was kicked out by the Thai senate, but anti-government protesters have remained on the streets and are now seeking to topple the government, which they say acts as a stooge for Thaksin.
Addressing a large anti-government rally at the capital's Democracy Monument, protest leader Satit Wongnongtaey hailed the strong turnout for so-called "People's Day".
"How can this government survive? How can the Thaksin system survive?" he said to applause from the crowd, many of whom came to the capital from the provinces.
In addition to the amnesty failure, Yingluck's ruling Puea Thai party was battered further by a Constitutional Court ruling last week that scuppered plans for a fully elected senate.
Pouncing on the defeats, the opposition Democrat Party, which is driving the anti-government protests, has lined up a battery of challenges to the government. Yingluck, amid calls to resign, faces a no-confidence debate this week.
But government-supporting "Red Shirts" have vowed to bolster Yingluck's embattled administration.
"Red Shirts also have to show our strength to protect democracy," said the group's leader Thida Thavornseth in a televised address Saturday.
"We will hold a peaceful rally and we do not want confrontation, so if there is violence it will not be ignited by Red Shirts," she added.
Thailand, which has seen 18 actual or attempted coups since it became a constitutional monarchy in 1932, has appeared irreconcilably polarised over Thaksin.
The telecoms tycoon-turned-politician draws ardent support from many of the country's rural and urban working class, but is loathed among the elite and middle classes, who accuse him of corruption.
Puea Thai swept to power in 2011 on a wave of support for Thaksin after a bloody military crackdown on the 2010 mass Red Shirt protests by the then Democrat-led government left scores of people dead.
The government's failed amnesty plan also angered many Reds, as it would have absolved those responsible for the violence.
Thida called on the group to rally Sunday in a stadium in the city suburbs in the name of "those who have sacrificed their lives".
The opposition-led group have said they will march to 12 different locations in the city on Monday, but have not given further details.