Tension gripped Kerala as an unidentified person allegedly hurled an explosive substance at the state headquarters of the ruling Communist Party of India-Marxist on Thursday night.
Police said the explosive substance was hurled at the AKG Centre, situated in the heart of Thiruvananthapuram, 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.
Police were alerted immediately and a team of top officials inspected the spot and CCTV cameras were scrutinised.
A bomb squad also inspected the spot.
CCTV visuals released by the CPI-M through the official media group of the AKG Centre showed a man reaching the spot on a motorbike and hurling the 'bomb' at the building and fleeing the spot.
The explosive allegedly hit the stone wall of the AKG Centre.
Senior CPI-M leader and Left Democratic Front 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.
The opposition Congress, however, rejected the allegations.
Meanwhile, many Kerala ministers, including Finance Minister K N Balagopal, Education Minister V Sivankutty, rushed to the spot.
Several workers of the CPI-M took out a protest march in Thiruvananthapuram.
The workers also protested in Pathanamthitta and other places in the state.
As tense situation prevailed, the police tightened security around offices of the Congress in the state.
CPI-M state secretary Kodiyeri Balakrishnan called for a peaceful protest.
Accusing the Congress-led UDF of deliberately creating provocation, Balakrishnan urged the party workers not to fall in their trap.
An investigation has been launched to identify the culprits behind the attack, police said.
The incident comes ahead of Congress MP Rahul Gandhi's visit to his constituency Wayanad on Friday.