Marxist leader VS Achuthanandan on Tuesday left for Kudankulam in Tamil Nadu to protest against the nuclear project but was stopped by police on the Kerala-Tamil Nadu border.
The former chief minister had set out for the coastal site in defiance of a fiat from his Communist Party of India-Marxist (CPI-M) and a request from the Kerala home department not to go to Tamil Nadu.
Achuthanandan's vehicle was stopped by the Tamil Nadu police at the border, about 30 km from Thiruvananthapuram.
Alighting from the vehicle, Achuthanandan said: "I am not interested in creating a law and order issue in your state (Tamil Nadu).
"(People have) been protesting in Kudankulam for the past 400 days, and my visit was to give moral support.
"Now that you are saying I cannot go, I accept your word and am returning back," he told the Tamil Nadu officials.
Achuthanandan, who heads the opposition in the Kerala assembly, also hit out at nuclear power, calling it "dangerous to mankind".
A large police force from Tamil Nadu was deployed at the border. They made it clear that if Achuthanandan defied them, he would be arrested.
CPI-M leader MM Lawrence earlier said Tuesday: "(Achuthanandan's trip) amounts to a serious breach of party discipline and he is inviting disciplinary action."
The Nuclear Power Corp of India Ltd (NPCIL) is building two 1,000 MW reactors with Russian help in Kudankulam. These have run into protests from locals who fear a nuclear disaster.