Campaign group Republic is to stage its first major anti-monarchy demonstration in London since its members were arrested on the day of the King’s coronation.

A few hundred people are expected to gather near the House of Parliament during the first state opening of the King’s reign on Tuesday.

The Metropolitan Police was criticised after six Republic members were detained ahead of a pre-agreed coronation protest on May 6.

Chief executive Graham Smith, who is now taking legal action against the Met, was among those held for more than 14 hours under the sweeping powers of the new Public Order Act.

Mr Smith said Tuesday’s protest was about the place of the Crown in parliament and the “ramshackle state of our constitution”.

“Our parliament is opened by a king wearing a crown while sitting in the Lords,” he said.

“It is just weird we continue with this pantomime that celebrates the worst, least democratic aspects of our political system.”

Republic has experienced an unprecedented growth since the coronation, Mr Smith added.

“Our continued programme of protests serves to remind people of the need for a democratic alternative to the monarchy,” he said.

“The monarchy is wrong in principle, corrupt and secretive in practice, and bad for British politics.

“It’s time it went and polling shows an increasing number of people agree.”

Republic is set to gather near parliament from about 8.30am.

Members of the group were detained on the day of the coronation after legislation came into effect days before the event, creating new offences of locking on or going equipped to lock on under the Public Order Act.

Mr Smith had discussed the planned demonstration for four months with senior Met staff but says he was physically prevented from calling the group’s designated liaison officer when he was stopped and arrested on the day.

On May 8, they were told no further action would be taken.

Mr Smith wants the Met to admit the arrests were unlawful and is seeking damages and costs.

A spokesman for the force has said: “We can confirm that a judicial review claim has been issued and it would be inappropriate to comment on ongoing proceedings.”