British police arrested over 20 people on suspicion of terrorism offences after they showed support for the newly banned Palestine Action group in London, officials said on Saturday, hours after the proscription came into effect.

The protest group was officially classified as a terrorist organisation from midnight on Saturday after the measure received overwhelming support from MPs and following the failure of a last-minute legal challenge to delay it.

Palestine Action supporters have defied a terror ban hours after it came into force. Several dozen activists gathered in front of Parliament to show their backing for the group, despite it being against the law to display support for a proscribed organisation.