LONDON (AP) — Hundreds of British police took part Tuesday in a major counter-terror exercise in London, testing the abilities of emergency services to respond to a Tunisia-style attack by extremists 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 said the Met has never before put on an exercise of this magnitude and that it would be "very realistic."

"Sadly, London is no stranger to terrorism," she said. "Given the changing nature of the very real terrorism threat and events around the world we need to constantly adapt our plans and prepare for new or emerging threats."

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

The exercises were taking place at a disused subway station.