The CBI on Monday filed an FIR against a dozen senior leaders of West Bengal's ruling Trinamool Congress and an IPS officer in the Narada sting footage case, raising the political temperature in the eastern state.

The opposition launched an all-out attack against the Trinamool, with the Left Front bringing out a large number of rallies demanding the resignation of the government.

However, Chief Minister Mamata Banerjee said mere filing of FIR does not mean the accused are guilty. "It is a political game, and has to be tackled politically," she said.

A Central Bureau of Investigation spokesperson said the list of FIR includes all 12 persons who were purportedly seen receiving money in the Narada sting operation footage. "Trinamool Congress MP Aparupa Poddar's name is also included in the list of FIR," the spokesperson said.

Among those whose names figure in the First Information Report are Trinamool vice president and Rajya Sabha member Mukul Roy, the party's Lok Sabha members Sougata Roy, Sultan Ahmed, Kakoli Ghosh Dastidar and Prasun Banerjee.

Also featuring in the list are state Ministers Subrata Mukherjee, Firhad Hakim, Suvendu Adhikari, city Mayor and Minister Sovan Chatterjee, legislator Iqbal Ahmed and former minister Madan Mitra.

IPS officer SMH Mirza was the other person named in the list. He was seen in the sting footage and the Narada website claimed he described himself as a "key person who collects funds for the Trinamool Congress".

The accused have been charged with criminal conspiracy and under various sections of the Prevention of Corruption Act.

"CBI conducted the Preliminary Enquiry into the said allegations expeditiously. Enquiry revealed prima facie material for registration of a Regular Case (FIR) under the charges of ciminal conspiracy to commit a crime (IPC 120B) and Section 7 and 13(2), 13(1) (a) and (d) of Prevention of Corruption Act, 1988. Further investigation is continuing," a CBI press release said.

The Calcutta High Court ordered a CBI preliminary inquiry into the case exactly a month back, on March 17, and asked the federal investigation agency to submit the report within 72 hours.

The Trinamool appealed to the Supreme Court on March 21, challenging the High Court' order.

The apex court refused to interfere with the high court order, but extended the deadline for the preliminary probe to one month.

On Monday, Narada News CEO Mathew Samuel welcomed the CBI move, saying the agency's action proved that the visuals in the sting footage are not doctored and there was no conspiracy behind the sting operation.

"For these reasons they have come to the conclusion to file an FIR," said Samuel, who had carried out the sting operation on behalf of the news portal.

The opposition tore into the ruling party.

State Congress president Adhir Ranjan Chowdhury targeted the chief minister for having earlier claimed that no such scandal has taken place.

"She needs to be asked whether she still feels false cases have been lodged against the Narada accused?" he asked.

Senior Communist Party of India-Marxist leader and noted lawyer Bikash Bhattacharya said the latest development was inevitable.

"This shows the big role that money plays in politics of Bengal, or entire India. We hope there will be no political interference impeding the CBI probe," added Bhattacharya, who had fought the case for the petitioners seeking a CBI probe into the case.

State Bharatiya Janata Party President Dilip Ghosh said the filing of FIR was expected.

"People of whole West Bengal were waiting for this. The probe is proceeding in the right direction," he said.

The controversy erupted in election-bound West Bengal in March last year when Narada News portal uploaded video footage purportedly showing the Trinamool leaders receiving money in exchange of favours to a fictitious company.

Three PILs were filed seeking a CBI probe, and the opposition went all guns blazing on the issue. But the Trinamool managed to retain power, and soon after Banerjee ordered a police probe to find out the "conspiracy" angle to the sting.

However, the Calcutta High Court ordered an interim stay on the police probe.