About 500 lawyers marched to Parliament on Tuesday, hours before the law was due to be debated.
They chanted “freedom to assembly” and “freedom to the people”, before police stopped most of them from entering the complex.
They say the government is trying to rush through the law without proper consultation.
“We hope the government will listen to the voice of the people,” said Bar Council President Lim Chee Wee, according to the Associated Press news agency.
Last week, the government announced the repeal of three emergency proclamations providing for detention without trial.
Much of the legislation dates back to the 1960s and 70s when Malaysia was racked by tensions between Muslim Malays and the ethnic Chinese, which at one point exploded into bloody riots.
Mr Najib also promised in September to repeal another law, the Internal Security Act, which has its origins in the anti-Communist legislation of the British colonial period and has been used for decades to detain and intimidate government critics.