Thirty animal rights activists were arrested Tuesday in Britain, Belgium and the Netherlands on suspicion of crimes targeting animal research, British police said.

Hampshire police said 15 men and 15 women faced questioning. Three were released by midday.

None of those arrested was immediately charged with an offense.

The sweep was, "one of the largest, if not the largest, police operation that has targeted animal extremism in the U.K.," said Assistant Chief Constable Adrian Leppard of Kent Police. He said the police were investigating "a range of criminal offenses including burglary, conspiracy to blackmail and offenses under the Serious Organized Crime and Police Act targeting animal research organizations."

In recent years extremists have attacked facilities and assaulted people associated with experiments on animals. Three years ago, activists dug up the body of a woman related to the operators of a guinea pig farm.

"The victims of animal rights extremism are not only companies or universities," Leppard said. "It is employees along with their families, their friends and neighbors who often are targeted in their own homes."