Emergency teams in bio suits descended on the Israeli embassy in a training exercise for a hazardous chemical incident.
A cordon was put in place and decontamination tents were erected at the back of the building in central London, yards from Kensington Palace.
Paramedics in full bio suits treated "casualties", played by members of the Royal Military Police, who were put through decontamination showers.
Police, paramedics and fire crews all took part in the event on Sunday to test the emergency service reaction to an attack or accident involving a toxic substance.
Commander Adrian Usher, head of the Met's Royalty and Specialist Protection, said heavy snow and icy conditions were an added challenge for the crews.
"The weather, which is exactly the sort of variable you can't predict, gave us challenges in terms of the number of staff that we could actually deploy from all of the emergency services," he added.
"Getting people to the scene was really tricky but that is exactly the sort of thing that throws up challenges that we would never plan for."
He said it is "absolutely vital" that emergency services can practise for a real crisis.
"Of course it's exactly the sort of exercise that everybody is praying never happens for real but if it does we take what we've learned from today and it puts us in a far, far better place to react should something like this happen in the real world," he added.
"A hazardous substance incident could have been caused by all sorts of reasons in the real world - some of them criminal, many not."
He continued: "Throughout this year we have seen many occasions when the emergency services have been tested for real and (this sort of exercise) has proved a really, really vital part of delivering these things safely for the public."
The exercise was planned months in advance and was not held in response to any specific threat.
It comes at a time of tension in the Middle East over US president Donald Trump's announcement that he is recognising Jerusalem as the capital of Israel.
Commander Usher said: "This was not in any way connected with geo-political events - we are very, very grateful to the Israeli embassy to have agreed to step up and take part in this, to allow us to practice at a protected secure premises."
Israeli ambassador Mark Regev said: "This is an example of the cooperation between this embassy and the emergency services.
"It's a good manifestation of the sort of good and healthy cooperation we have and represents a very positive relationship we have between the two countries."