By Ed Cropley

BANGKOK, Nov 23 (Reuters) - Thousands of anti-government protesters rallied in central Bangkok on Sunday, the start of what they call the "final battle" in a five-month street campaign to oust the administration.

The crowd led by the People's Alliance for Democracy (PAD) planned to march to parliament early on Monday, where police have erected barricades and stationed trucks with water cannons.

A similar protest in October left two people dead and hundreds wounded, including scores of police, when police fired tear gas to break up the rally aimed at disrupting parliament.

Whatever happens, Thailand is likely to remain divided between the rural and urban poor who support Thaksin Shinawatra, ousted as leader in a 2006 coup, and the Bangkok middle and upper classes, loosely represented by the PAD, who despise him.

The elected administration, which is accused of being a Thaksin puppet, has been working out of temporary offices in an old airport since the PAD overran Government House in August.

It has all but given up on policy-making, intensifying investors' concerns about the export-dependent economy's ability to cope with global recession.

The following scenarios examine what might happen next:

MESSY SUNDAY

- Unless one side backs down -- and there are few signs they will -- Sunday/Monday could be messy. The PAD's stated intention is to trigger a coup and anarchy is its main weapon.

Police will be mindful of last month's high number of casualties, especially the protesters who lost limbs from exploding tear gas grenades, and are likely to be more cautious.

However, hardline PAD elements are armed, and shot at police lines last month. If any officers are shot dead, it is not hard to see their colleagues responding in kind.

RECRIMINATIONS FLY, CRISIS RUMBLES ON

- Whatever happens on Sunday/Monday, accusations will fly from both sides, ensuring the crisis rumbles on amid a poisonous political atmosphere.

There is bound to be a small lull around the king's birthday on Dec. 5, but tensions will rise ahead of Dec. 13 when the exiled Thaksin holds a "phone-in" to a sports stadium rally.

A similar gathering of 40,000 people on Nov. 1 ended without incident but the venue then was on the outskirts of the city. The Dec. 13 rally is going to be just 1.5 km (1 mile) from the PAD protest site, increasing the chances of confrontation.

COURTS DISBAND RULING PARTY

- The Election Commission has already found the ruling People Power Party (PPP) guilty of vote buying and the Supreme Court is expected to endorse the decision in December or January, leading to the party's immediate dissolution.

Top figures such as Prime Minister Somchai Wongsawat, Thaksin's brother-in-law, will be barred from politics but most MPs will simply switch to a "shell" party already lined up, and as long as the ruling coalition holds together, it will stay in power.

Even if there was an election, the ex-PPP would be likely to win due to solid rural support for Thaksin.

Emotions will run high as the court ruling nears.

PAD CAMPAIGN FIZZLES OUT

Before this week's grenade, polls showed waning support for the PAD, which has been snarling up Bangkok traffic for six months, and its numbers at Government House have dwindled.

However, it is inconceivable it will simply wither and die, especially as it has the explicit backing of the highly influential Queen Sirikit. Protesters also made clear on Thursday they were undeterred by the threat of more grenades.

MILITARY COUP

It is never wise to rule out a coup in a country that has had, on average, one successful or attempted putsch every four years since the overthrow of absolute monarchy 76 years ago.

Army chief Anupong Paochinda has put public pressure on Somchai to quit, but has also said the army will not seize control as it is powerless to heal the basic political rift.

KING INTERVENES

- Regarded as semi-divine by many, King Bhumibol Adulyadej carries huge informal political clout and in six decades on the throne has intervened in several disputes, favouring at various times both elected and military administrations.

However, the 80-year-old has stepped in previously only after major bloodshed, and his advancing years and deteriorating health raise doubts about his ability to calm any new outburst. (Editing by Darren Schuettler and Bill Tarrant)

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