Police will use the new Serious and Organised Crime Act, which came into force today, to seek orders that particular gangs be "declared," essentially making it illegal for members to meet or gather together.

Assistant Commissioner Tony Harrison said officers had been working for some time to prepare cases against key groups, with bikie gangs the first targets.

The new laws also allow magistrates to issue control orders against gang members and introduce a new offence of criminal association, to stop gang members and their hangers-on from gathering.

Mr Harrison said he expected the first applications for declarations to go to Attorney-General Michael Atkinson soon.

"What we have been doing is considering all of the outlaw motorcycle gangs and their respective criminal activity," he said.

"We've been going through a fairly comprehensive process in relation to consolidating our information and it won't be too far down the track that we will be moving forward and making an application for the declaration process."

Police intelligence suggests there are about 250 initiated members of eight bikie gangs operating in South Australia, with a further 150 closely associated.

Mr Atkinson said the new laws were a clear message that those people were not welcome in South Australia.

"They should either leave their gangs or leave the state. We will pursue them until they do," he said.

"Members of these gangs are responsible for dealing drugs, using deadly weapons, murders and countless assaults, not to mention the blackmail and intimidation these bullies inflict if they feel threatened."

Under the new laws, police must satisfy the attorney-general that members of declared gangs associate for criminal purposes and are a risk to public safety and order.

Mr Atkinson said the laws were a world-first.

Police said the legislation had attracted interest from other jurisdictions, both in Australia and overseas.

Legal challenges were expected but the government was confident the new provisions would stand up in court.

But the state opposition accused the government of moving too slowly. Police spokesman David Ridgway questioning why it had taken three months to enact the legislation following its passage through parliament.

"In the past three months there have been bikie-related shootings, a murder, home invasions, raids on drug labs and cache of illegal firearms uncovered," Mr Ridgway said.