Hours after an unidentified person allegedly hurled an explosive substance at the state headquarters of the ruling CPI(M) here, the opposition Congress on Friday categorically rejected the Marxist party's charge that they were behind the incident and said attacking the political opponent's office was not their party's policy.

In his reaction, Leader of Opposition in the state Assembly, V D Satheesan said the party leadership had no knowledge of the midnight incident and would let the police investigate and ascertain the truth.

Alleging that the people of the state clearly know that the Congress party and its workers were the ones who plan and execute such attacks, he asked the ruling party to explain on what grounds they were levelling charges against them.

"The CCTV visuals were not clear... Let the police probe and find out the real culprits.. The Left government is facing a series of allegations and they are on the defense due to our ongoing protests. Those who want to shift the attention and focus from the present issues are the ones behind the attack," he told reporters.

Asked whether he was suspecting that CPI (M) itself was behind the attack, Satheesan did not give a clear reply.

"It is an important day for us due to Rahul Gandhi's Kerala visit. Do you think we would attack the AKG Centre and shift the focus from the present issues being faced by the government?" the Leader of Opposition asked. KPCC chief K Sudhakaran also rejected the Marxist party's allegation and accused LDF Convener E P Jayarajan of 'masterminding' the attack against the party headquarters.

Meanwhile, security has been beefed up at the party offices of the Congress and CPI(M) in various places in the wake of the incident.

Tension gripped Kerala as an unidentified person allegedly hurled an explosive substance at the state headquarters of the ruling CPI(M) Thursday night. Police said the explosive substance was hurled at the AKG Centre, situated in the heart of the capital city, by a motorbike-borne man at around 11.30 pm.

CPI(M) leaders, who reached the spot alleged that it was a "bomb attack". Some party leaders, who were staying at the AKG Centre, said they heard a powerful blast outside the building.

Senior CPI(M) leader and LDF convener E P Jayarajan, who stays in the neighbourhood, alleged that the Congress was behind the provocative action and appealed to the CPI(M) workers to remain calm.