Police grappled with protesters in Bangkok on Friday as thousands of demonstrators marched through the Thai capital in an attempt to force the five-month old government from power.

One column moved up to a barricade where they faced off with a phalanx of riot police five-deep, pushing and shoving before sitting down in the middle of a road near the prime minister's office.

Thousands of police backed by water cannon and tear gas have been deployed to stop the protesters, whose four-week campaign has increased political tensions at a time of stuttering economic growth and soaring inflation.

The People's Alliance for Democracy (PAD), a motley collection of businessmen, academics and royalists united by their hatred of former leader Thaksin Shinawatra, say hundreds of thousands should join their "D-Day" assault on Government House.

Police expect only 14,000 demonstrators to turn up, but say they are ready to use force against the marchers, most of whom are middle-class Bangkok residents who view the government elected in December as an illegitimate Thaksin proxy.

Fears of clashes last month between police and demonstrators stoked rumours of another military coup less than two years after the army's bloodless removal of Thaksin, who insists he has retired from politics although few believe him.

Metropolitan police chief Lieutenant-General Aswin Kwanmuang said leaders of the PAD, whose 2005 campaign against Thaksin ultimately led to the 2006 coup, should abandon their aim to lay siege to Government House.

"We urge the PAD not to damage the country further. Political problems should be solved in parliament, not on the street," he told reporters before the march began.

At least eight barricades of crash barriers and prison trucks, manned by thousands of police drafted in from all over the country, prevented all access to the government compound, which sits in the heart of Bangkok's leafy "Old Quarter".

TROUBLEMAKERS

Although the PAD says it is staging a peaceful rally, dozens of its bodyguards have armed themselves with baseball bats, plastic helmets and wooden shields, some emblazoned with pictures of revered King Bhumibol Adulyadej.

PAD co-leader Chamlong Srimuang, a retired major-general who led a 1992 "people power" protest against a military-led government, said they would defend themselves if government supporters attacked the marchers.

"We have some baseball bats because we don't believe the police can protect us from those troublemakers from hell," Chamlong told the crowd.

Since last night several hundred government supporters have held their own rally close to the PAD stage, the two sides firing insults at each other as a few dozen police kept them apart.

The stock market edged higher on bargain hunting on Friday, while the baht weakened further due to the political uncertainties and dollar buying by importers, dealers said.

Thailand's benchmark stock index has dropped more than 13 percent since the PAD started its campaign on May 25, as investors worried about rising tensions at a time of stuttering economic growth and soaring inflation.

Army chief Anupong Paochinda, a member of the military council that ousted Thaksin, insisted the army would not get involved, perhaps mindful of unrest in 1992, when soldiers opened fire on pro-democracy demonstrators, killing dozens.

"Politics must be resolved by political means," Anupong said.

So far, the PAD campaign has only managed to muster crowds of a few thousand.

However, union leaders at the Electricity Generating Authority of Thailand, the main state power provider, said this week their members would take leave to join the rally, suggesting its numbers may be higher.