BANGKOK - Tens of thousands of anti-government demonstrators massed in Bangkok on Sunday, with a major rally scheduled later by their rivals, as Thailand faces its most significant political street action since bloody protests in 2010.
Organisers said 100,000 anti-government protesters had gathered by Sunday afternoon, while thousands of pro-government "Red Shirts" were expected to mobilise later at a suburban football stadium in support of Prime Minister Yingluck Shinawatra's crisis-hit administration.
Both groups have vowed to remain in the capital overnight as tension rises in a city which has seen several bouts of unrest since Prime Minister Thaksin Shinawatra - Yingluck's brother - was deposed in a military coup seven years ago.
Authorities said they expect opposition protesters to try to march on parliament and Government House on Monday, but vowed to show restraint.
"We will not use force and we will try to avoid any casualties," police spokesman Piya Utayo told AFP, estimating the anti-government rally to be at least 30,000-strong.
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 rejected by the senate, but anti-government protesters have remained on the streets and are now trying to topple the government, which they say acts as a stooge for Thaksin.
Aerial footage showed tens of thousands of protesters crammed into the streets leading to the city's Democracy Monument, which has become the focus of the anti-government protests.
Addressing the rally, protest leader Satit Wongnongtaey hailed the strong turnout for the so-called "People's Day" rally, with many demonstrators arriving from the provinces.
"How can this government survive? How can the Thaksin system survive?" he said to applause from the crowd.
In addition to the botched amnesty bid, Yingluck's ruling Puea Thai party was battered further by a Constitutional Court ruling last week that scuppered the party's 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 - although her party dominates the lower house and should comfortably defeat any motion.
Government-supporting "Red Shirts" have also vowed to bolster her embattled administration.
"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," said the group's leader Thida Thavornseth in a televised address Saturday.
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 by the elite and the 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. Scores of people were killed.
The government's failed amnesty plan also angered many Reds, because it would have absolved those responsible for the 2010 violence.
The opposition-led group have said they will march to 12 different locations in the city on Monday, but have not given further details.