Hundreds of British police are taking part in a major counter-terror exercise in London, testing the abilities of emergency services to respond to a Tunisia-style attack of terrorists with firearms.

The exercise dubbed Operation Strong Tower has been planned for six months and is not connected to the recent attack at a north African beach resort. Deputy Assistant Commissioner Maxine de Brunner says the Met has never before put on an exercise of this magnitude and that it would be "very realistic."

The Charlie Hebdo attack in Paris and the hostage-taking siege in Sydney had an influence on the test, which involves 14 different organizations and will last Tuesday and Wednesday.

The exercises are taking place at a disused subway station.