Thousands of protesters at the Democracy Monument and in Sanam Luang area started their marches to 13 government agencies and television stations at 8.30 am today.
The enormous crowd is backing opposition Democrat Party and its allies, to put pressure on the ruling Pheu Thai-led coalition government. Their aim is initially to shut down, and eventually end, the so-called "Thaksin regime".
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Two key groups of anti-government protesters are expected to move across the capital today.
One group will march to block access to Government House and Parliament, while the other will pressure government agencies, a high-ranking protest source told the Nation daily.
Political deadlock is expected to hit the Yingluck government if the Parliament is blockaded; such an action would render the announcement of a last-resort House dissolution impossible.
The centre has recommended protection of strategic locations, at the highest level, to deter any attempts to blockade them, but it is doubtful whether police would be able to stop crowds, which outnumber them, from doing so.
Estimates of the number of protesters yesterday varied, with security sources citing a figure of around 100,000 and a spokesman for the Democracy Monument rally putting the number at 440,000 in the afternoon with more expected in the evening.
The pro-Thaksin "Red Shirts" gathered yesterday at Rajamangala Stadium - a considerable distance from the anti-government protests.
A massive security net has been cast over the Thai capital as former opposition Democrat MP Suthep Thaugsuban yesterday gave a call to "root out the Thaksin regime".
War rooms set up at the Supreme Command are monitoring the protests round the clock, a military source said.
The source said military leaders feared that the protests would escalate out of control because the number of people gathered was so large.
Thailand has witnessed instances of sporadic unrest since former premier Thaksin Shinawatra - brother of Yingluck - was deposed in a military coup seven years ago.
The Thai capital has already been hit by opposition-backed protests in the past few weeks following a controversial amnesty bill that could have paved the way for Thaksin returning from his self-imposed exile.
The bill, which would also absolve those responsible for military crackdown on Red Shirt movement in 2010, was passed by the lower house but rejected by the upper house.
However, protests have not disappeared as the anti government supporters now want to boot out the government. But government-backed Red Shirts have vowed to bolster Yingluck's embattled administration.
Thailand has seen 18 actual or attempted coups since it became a constitutional monarchy in 1932.