The Government is planning a change in the law to allow police to arrest suspects without evidence, it was claimed today.
The Law Society said it believed the new powers would be included in the Bill which will create the new British FBI.
It warned that the cumulative effect of the Government's clampdown on crime and terrorism would be a step towards a police state.
Janet Paraskeva, the Law Society's chief executive, said: "The Government is in serious danger of overstating the threat to public order and national security and bringing in draconian new laws, which will take away centuries of hard won rights.
"If the Bill to establish the Serious Organised Crime Agency (Soca) includes the power to arrest someone without evidence, then solicitors could not support it.
"That would be a serious step in the direction of a police state.
"Anyone could be lifted from the streets or from their homes just on the basis of suspicion."
She added: "The threat to end jury trials for terrorism cases is another chipping away of the centuries old rights for people to be tried before a jury of their peers, which goes back to Magna Carta."
Soca will have around 5,000 investigators to crack down on serious crime and fraud, merging the National Crime Squad, the National Criminal Intelligence Service and the investigating arms of customs and the immigration service.
A Law Society spokesman said they had been told on good authority that the new powers of arrest were being considered by ministers.