Thiruvananthapuram, Oct 28 (IANS): At least 21 people have been taken into custody and cases registered against a thousand Left supporters for the attack on Kerala Chief Minister Oommen Chandy's convoy at Kannur Sunday, police sources said Monday.

Chandy suffered injuries to his forehead and chest after suspected Communist Party of India-Marxist (CPI-M) activists stoned his car here Sunday evening.

The incident occurred when Chandy was on his way to attend the annual Kerala Police athletics meet in Kannur.

Chandy was airlifted from Kozhikode last night and admitted to the Medical College Hospital in the state capital past midnight.

"This sort of action by a political party is not good for the smooth functioning of democracy," Chandy told reporters after arriving at the airport.

The National Students Union of India, affiliated to the Congress party, has called for a state-wide protest and most educational institutions in the state will remain closed.

Among those who have been charged for the attack are two CPI-M legislators C. Krishnan and K.K. Narayanan and three former legislators P.K. Sreemathi, P. Jayarajan and M.V. Jayarajan.

Doctors Monday morning said Chandy's condition is stable.

Though Chandy asked his party men not to observe a state-wide strike, shutdowns were reported at several places across the state.

The top leadership of the CPI-M has denied any role in the attack.

"We have no role in this attack as this is not our policy. The need of the hour is that the government should conduct a detailed probe and bring out the truth of what has really happened," said CPI-M politburo member S. Ramachandran Pillai.

At Kannur, which is the citadel of the CPI-M, the police have strengthened their presence after reports surfaced that there could be clashes between the Congress and CPI-M supporters.