Core leader Sondhi Limthongkul of the People's Alliance for Democracy (PAD) personally launched the march at 6am from Government House.

By 8am (0100 GMT) on Monday, an estimated 20,000 protestors had surrounded parliament, effectively besieging it.

Sondhi called it the PAD's "final battle". It climaxes a six-month campaign which began on May 25 and has featured protests 24 hours a day.

Besides parliament, the PAD targetted other key sites for protests on Monday - Police headquarters and the main office of coalition partner Chart Thai party among them.

Awaiting the march were hundreds of police manning steel barricades outside the Bangkok parliament building, with fire trucks parked nearby.

Across town at the Suan Kaew temple in Nonthaburi, members and backers of the pro-government United Front of Democracy Against Dictatorship heard equally strong speeches in defence of fugitive ex-premier Thaksin Shinawatra, and against the yellow-shirted PAD.

Crunch Day is Monday, when many fear that three events could coincide:

Tens of thousands of PAD protesters will try once again to besiege parliament and confront police whose duty is to secure a scheduled joint meeting of the House and Senate.

Hundreds or thousands of UDD supporters will ignore pleas from their leaders and civil figures to confront the PAD in the name of keeping parliament open.

Members of parliament will force a showdown by insisting on getting into the House, setting up a violent confrontation involving all three sides.

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