Thailand's anti-government "Red Shirts" threw tyres onto elevated city rail tracks Tuesday to try to thwart a feared military crackdown against their campaign for fresh elections.

As they beefed up their defences, the Reds said they would turn up the heat on Prime Minister Abhisit Vejjajiva by appealing Wednesday for more people to join their mass rally in the capital.

Bangkok's Skytrain was halted for several hours after Reds laid tyres on the rails at a station above the protest site to stop troops using it for a possible surprise attack, disrupting the morning rush hour commute.

Abhisit struck a moderate note in an interview with the BBC, saying he would step down if he believed he were an obstacle to stability, but that deeper-seated issues were at stake. There was no imminent likelihood of martial law being declared, he added.

However armed soldiers guarded the capital, which is under a state of emergency and has been rocked by the worst political unrest for almost two decades, with 26 people killed and almost 1,000 injured this month.

Security forces were ready to retake the Reds' rally area, but would only use their weapons in self defence, said army spokesman Sunsern Kaewkumnerd, who has previously warned protesters they could face real bullets in any crackdown.

"The manpower is ready. We are waiting for the right time," he said, adding that troops had set up checkpoints to prevent people bringing in weapons to the confrontation zone in Bangkok's commercial heart.

The government said it would not tolerate road blocks set up to prevent more troops heading to the capital by protesters who have been staging weeks of rallies to campaign for immediate elections.

Asked if he would step down if he believed he were an obstacle to Thailand's recovery and stability, Abhisit said in interview excerpts released by the BBC: "Of course. I’ve never put my interests above the country's."

But he said the issues were "not a purely political problem. There are security problems involved, there are terrorist problems involved.

"We need to tackle all those issues at the same time. When we talk about the stability everyone wants to achieve, not a short-term stability waiting for the problem, the same problem to be repeated again and again."

Abhisit has rejected an offer by the Reds to disperse if elections are held in three months' time.

A day after Thailand's revered king spoke of the need for peace in his first public remarks since the crisis began, protesters remained on high alert for a crackdown.

They have reinforced their roadblocks and stepped up security checks on the perimeter of the sprawling protest site, which has been fortified with barricades made from piles of truck tyres and bamboo stakes.

"Each passing day will make us stronger. At least we can hold government forces at bay for another day," said a Reds leader, Jatuporn Prompan.

The government has said it has uncovered a network of people suspected of plotting to overthrow the kingdom's revered monarchy and will issue summons and possibly arrest warrants for alleged members.

The Reds denied they were involved in any attempt to undermine the royals and threatened to sue the government over the accusation.

"This is the government's way of using its imagination to crush opponents," said protest leader Nattawut Saikuar.

Strict legal provisions in Thai law can land anyone guilty of insulting the royal institution with up to 15 years in prison.

The Reds are mostly supporters of ex-premier Thaksin Shinawatra, who was ousted in a 2006 coup and now lives overseas to avoid a jail term for corruption.

Thaksin, who was in Montenegro at the weekend, said he had been in contact with the Red Shirts.

"We just fight for democracy," he told reporters there.

- AFP