LONDON -- British police have expressed regret over the arrests of anti-monarchy protesters during the coronation of King Charles, saying they will not face criminal prosecution following criticism that the security response was heavy-handed.

London's Metropolitan Police said six people were arrested on Saturday under new powers from the Public Order Act, a law to restrict protests that came into force days before the coronation, the biggest ceremony seen in Britain for 70 years.

As crowds gathered, police arrested the leader of the anti-monarchy campaign group Republic and six of its members who were planning to take part in its biggest protest.

The incident cast a shadow over what had otherwise been a weekend of largely positive coverage for the Royal Family.

Police said protesters were arrested because they had items that could be used to lock them to positions on the coronation route. Republic said the items were for securing placards.

The force said an investigations team later examined the items and no further action would be taken. The police expressed "regret" the demonstrators had been prevented from protesting.

Graham Smith, Republic's chief executive and one of the six protesters arrested, said he previously told police about what type of material they would be carrying and where they would be.

"I believe that there was a pre-meditated decision before we even arrived to arrest us," he told Reuters. "If they were trying to diminish our publicity it has massively backfired.

"What has happened has gone around the world."

Smith said police had apologized to him in person on Monday but he planned to talk to lawyers about taking legal action.

Government ministers said police had faced a difficult situation in trying to prevent dangerous disruption.

The arrests were criticized by civil liberties groups.

"This is something you would expect to see in Moscow, not London," said Yasmine Ahmed, Human Rights Watch's U.K. Director.

(Reporting by Sarah Young and Andrew MacAskill; Editing by William Schomberg, William Maclean)