Meanwhile, Thai Rak Thai Party executive member Pairot Suwan-chawee yesterday accused the People's Alliance for Democracy (PAD) of trying to force Thaksin out of politics permanently by either harming him or even killing him.

Abhisit's call came amid growing public suspicion that last Thursday's arrest of an Army officer driving a carload of explosive materials was a politically motivated set-up to gain sympathy for the embattled caretaker PM.

Police and the ruling Thai Rak Thai Party have dismissed such suspicions and insisted the alleged assassination plot was real.

Thaksin said on Friday he had been informed that four retired and active military officers were behind the alleged bomb plot targeting him. However, he rejected speculation that his mentor-turned-critic, Maj-General Chamlong Srimuang, was involved.

Abhisit yesterday called for urgent action against the alleged masterminds.

"The prime minister said with confidence that four military officers were behind a plot to assassinate him. If he knows who they are, the government must arrest them straight away or the attempted assassination will remain a controversial issue with the public," Abhisit said at the Democrat Party headquarters.

He urged the government to be more cautious in its remarks and avoid exaggerating an issue that could affect the country's image.

Abhisit said Thai society had already been divided and the matter should not be played up for political gain.

He urged the relevant authorities to uncover the facts of the matter and not jump to the conclusion that last week's incident was an attempt on Thaksin's life.

Abhisit noted that when Thaksin first came to power in 2001, he had made a similar claim [about a plot to kill him] after a plane he was due to board mysteriously burst into flames on the tarmac at Don Muang Airport. An official investigation later found the explosion was an accident.

Democrat spokesman Ong-art Klampaiboon yesterday said police were too quick to conclude the discovery of the carload of explosives was an attempt to kill Thaksin. He said questions had been raised in many quarters that the incident was pre-organised and this suspicion was fuelled by the lack of public trust in Thaksin and his administration.

The spokesman said if this were true, it did not bode well for the country's international image or the domestic political situation.

He criticised Thaksin for cancelling his election campaign trips to the provinces in the wake of last Thursday's event.

"A person of the PM's stature has to show that Thailand is safe and peaceful. His decision to cancel his trips is harmful to the image of security in the country," Ong-art said.

The spokesman said key Democrats viewed the matter as a power struggle between opposing groups, with the police and military being used as pawns.

Mahachon Party leader Sanan Kachornprasart has also criticised the police for jumping to the conclusion that it was an assassination plot, even though their investigation has not been completed.

Pairot said it was obvious that those who wanted to oust Thaksin were responsible for the alleged assassination plot against the PM.

"The methods the PAD and their allies use is to make Thaksin take a political break for good and take his life by using a massive amount of explosives,'' Pairot said.

He claimed the PAD and Squadron Leader Prasong Soonsiri were part of a group that may have been involved in the alleged plot because they had rushed to discredit Thaksin's comments soon after the event.

"Why did Prasong jump to a conclusion [that it was a set-up]? Does he know something about this bomb?'' Pairot said.

He said the PAD accused the government of setting up the assassination plan as a stunt because it feared Thaksin would win public sympathy from the incident.

PM's Office minister Newin Chidchob will this week file a libel suit against leading government critic Sondhi Limthongkul for accusing him of helping to fake the assassination attempt on the caretaker prime minister, TRT deputy spokesman Chatuporn Prompan said yesterday.

During his "Muang Thai Rai Sapda" programme at Lumpini Park on Friday, Sondhi accused Newin of being behind the incident, saying the minister was "the worst director of this bomb hoax".

Chatuporn said that Newin was gathering evidence and had ordered the party's legal team to file the libel suit against Sondhi.

Meanwhile, Thai Rak Thai spokesman Sita Divari said police had found additional evidence at the house of bomb suspect Lieutenant Thawatchai Klinchana, which could help link him to other suspects involved in the alleged assassination plot.

Chatuporn accused Prasong of trying to distort the issue and denied the bomb plot was a government set-up to divert public attention from the widening chasm between government opponents and supporters.

He also denied his party was behind the gathering of Thaksin supporters outside the residence of Privy Council president Prem Tinsulanonda. The group called on Prem to help protect Thaksin's life.