Elite police officers played out a Paris-style attack in heart of the City in a training exercise to prepare for a terror attack on London.

Bursts of gunfire were heard from the office block in the Square Mile on Tuesday as officers dressed in gas masks and body armour brandished AK-47s in their roles as terrorists storming the building in a mock attack.

Armed and masked officers then entered the block in waves to clear the site of gunmen in the high level Met Police training exercise.

Dummy 'corpses' littered the ground to make the exercise more authentic. Officers also had to deal with a home-made bomb and the gun-wielding "terrorists" themselves.

The air in the building was spiked with the smell of gunpowder to add to the realism of the training.

Crews from London Ambulance Service and Fire Brigade also took part in the roleplay.

It comes as counter-terror officers were briefed to confront any Paris-style maurading gunmen in order to stop mass shootings.

Officers will "go forward" to engage with terrorist shooters even if it means leaving casualties untreated as the attackers are pursued.

The training aims to make the most of the first aid "golden hour" after someone is injured. The exercise was part of annual training for the more than 2,000 authorised firearms officers in London.

They were briefed only that terrorists had stormed a shopping centre before entering the building and searching every room and corridor to make sure it was safe.

Assistant Commissioner Patricia Gallan said: "We are asking [our officers] to do a different thing for what they did previously and it's not about standing back – it's about going towards the threat.

"There may be casualties and in meeting that threat they will have to walk over casualties that might have been injured and wait until it is safe for someone else to go and help them."

"In asking them to go forward we are asking them not to give first aid to people that might be injured, and it might well be their colleagues as well."

She added: "But the most important thing is to actually get to the threat and stop the killing of additional people and that's why we have to keep going forward and not tending those who are injured at the time.

'We would go as quickly as we can but as safely as we can. It is a tragic set of circumstances if it happens, but as quickly as possible we will get the ambulance service in."

Officers who patrol in armed response vehicles were first at the scene, followed by highly trained specialist counter-terrorism marksmen. They spent a number of hours making the building safe.

Although it bore chilling similarities to the attacks in Paris in which 130 people were killed by Islamic State-supporting terrorists, the Met said the scenario was actually written months ago.

David Videcette, who was in the Met’s counter-terror unit between 2004 and 2010, told the Standard the force is now in a much stronger position to deal with threats to London.

He said Tuesday’s exercise would have been set up long before the Paris attacks, adding: “It’s more effective than anything we dealt with. Back in 2004 and 2005 a lot of training was based around dealing with explosive devices, but not particularly around the safety of people.

“The threat has changed. We’ve been expecting something like Paris for many years to be honest, it’s been on the radar for many years. Terrorists are moving away from explosive attacks.”

Mr Videcette, who now works as a security consultant for high-value individuals, added of the Met: “The training they are doing is fantastic, it puts the police in a really good position.

“In terms of prevention, we’re much better prepared than anyone on the continent to deal with this sort of thing.

“Our intelligence is very very good. The relationship between the police and the security services is very closer – much closer than in 2004 and 2005.

“We’re better placed to stop it [a Paris-style attack] ever taking and stopping people slipping through the net. We’ve got a very, very well-trained counter terror unit.”