Under its revised plan, the Saturday Anti-coup Group would "alert Prem" if its campaign managed to collect 30,000 signatures and would rally to demand Prem's resignation once the number of signatures reached 50,000.

The petition will be presented to His Majesty the King if the group gathers 100,000 signatures.

At last Friday's campaign launch, the group set a target of 200,000 signatures in a week and planned to petition via the Office of His Majesty's Principal Private Secretary.

In a measured reaction, authorities cautioned that the campaign was inappropriate, because it could be construed as an attempted transgression on the royal prerogative to appoint a Privy Council.

Delivering a rebuttal, group spokesman Wipu Phumipatana-thai said his group had no intention of offending the monarchy.

"The Saturday Group is a victim of a smear campaign by the military dictatorship, which often tries to portray political opponents as committing lese majeste," Wipu said.

Commenting on claims that his group received financial backing from the ousted government, he said he wanted to invite the public, including ousted premier Thaksin Shinawatra and the military, to contribute funds "to advance democracy".

The group's key member, Nopporn Namchiangtai, said they had the right to petition His Majesty to air grievances inflicted by the royal entourage.

Nopporn said he did not view the signature campaign as an offence against the monarchy, because his group was prepared to respect any royal decision on the issue.

Acting national police chief General Seripisut Temiyavej said he viewed the signature campaign as containing malicious remarks against Prem.

"Police are ready to launch an investigation into alleged libel if Prem steps forward to complain as the injured party," Seri said.

He said police were studying "pertinent" legal issues and would complete their report in a few days, which would help him form a decision on how to deal with the signature campaign.