He said Mulayam Singh is talking "bakwaas" (rubbish).
Lalu Prasad said that Mulayam is not aware of what he was talking and what will be its impact.
Capital punishment to rapists is justified, the Rashtriya Janata Dal chief said.
At an election rally in Moradabad in Uttar Pradesh, Mulayam Singh also said that if his party leads the government at the centre, he would change the law.
"Death penalty for rape is unfair, boys make mistakes... will change rape laws when we come to power," Mulayam Singh said.
Three men convicted for the repeat crime of gang rape in the Shakti Mills complex in Mumbai have been sentenced to death by a court, invoking the amended laws against rape.