A huge counter-terror exercise was held over one Kent town to test the region's response to an attack.

Two black helicopters swooped over Gravesend at various points on Tuesday and Wednesday.

The craft were then spotted landing on the top platform of the hulking drill ship Sertão, which is docked at the old Tilbury power plant site while it awaits sale.

Essex Police were involved in the run throughs but said partners joined them as they prepared for the worst at the power station and DP World London Gateway, a dock in Stanford-le-Hope.

The operation was cloaked in secrecy, with police unable to provide information while it was going on.

It saw a firearms incident develop with police along with other emergency responders putting in place response plans to defuse the threat. Role players were used to make the exercise as close to real-life as possible.

Police advised people not to approach the sites or take any footage of the tactics being used.

Essex Police assistant chief constable Pippa Mills, who led the regional exercise on behalf of policing in the eastern region, said: “We work together to run routine exercises like these because it is important that we test out our ability to deal with large scale incidents, both as police officers but also how we work collectively as emergency responders. I would like to thank DP World, local businesses who may have been impacted by the activity as well as the local community for their support.

“The exercise was not based on any threat or intelligence however recent national and international events continue to highlight the importance of public organisations testing their emergency response procedures to assure the safety of the communities we serve.”

In an unrelated incident Essex police's maritime unit intercepted a man on a jetski after he had been spotted taking pictures of Town Pier as the Tilbury Ferry motored past.

He shot off after snapper Fraser Gray started taking photos of him but was soon collared by officers who told him to leave the river.

Port of London Authority spokesman Martin Garside said: “An individual was spotted using a jetski near Gravesend. He was intercepted by a police boat and advised very clearly that jetskis are not permitted anywhere on the tidal Thames in this area.

“He was further advised to remove his craft promptly from the area – an instruction which he complied with.”