BANGKOK - Tens of thousands of pro- and anti-government demonstrators massed in rival rallies in Bangkok on Sunday, as Thailand grappled with the most potent revival of street politics since bloody protests in 2010.
By Sunday evening organisers said more than 100,000 opponents of Prime Minister Yingluck Shinawatra and her crisis-hit administration had gathered in the city's historic centre, calling for her government to be toppled.
Police earlier in the day estimated the crowd as at least 30,000.
At the same time thousands of pro-government "Red Shirts" began arriving at a suburban football stadium in support of Yingluck, with up to 30,000 people expected.
Both groups have vowed to remain in the capital overnight as tension rises in a city which has seen several bouts of unrest since former premier Thaksin Shinawatra - Yingluck's brother - was deposed in a military coup seven years ago.
Police said they were preparing for opposition supporters to march on parliament and key government buildings late Sunday or Monday, despite special security laws being enforced in sensitive zones.
"We will not use force and we will try to avoid any casualties," police spokesman Piya Utayo told AFP, appealing for the peace to hold.
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 would have pardoned those responsible for a deadly military crackdown on his Red Shirt supporters.
The bill was rejected by the senate, but 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 people crammed into the streets leading to the city's Democracy Monument, which has become the focal point of boisterous but peaceful anti-government protests.
Addressing the "People's Day" rally, protest leader Satit Wongnongtaey hailed the big turnout.
"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 buffeted 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 faces a no-confidence debate this week - although her party dominates the lower house and should comfortably defeat a move against her.
She called for the demonstrations to remain peaceful and within the law.
"The government does not want to see conflict which will lead into violence," she said on her official Facebook page, adding the country had not reached a "dead-end" despite the revival of street politics.
Government-supporting "Red Shirts" have vowed to stand by her embattled administration, accusing the opposition of trying to provoke intervention by the kingdom's powerful army.
"It's not really important how many anti-government people there are," the group's leader Thida Thavornseth told AFP.
"What matters is if they try to do something violent, that could change the situation," she said.
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.