But there are other key rallies taking place in Belfast, Edinburgh, Glasgow and Aberdeen, and at two US military bases in Gloucestershire and Yorkshire.
Police called in
The London rally is being organised by the Stop the War Coalition. Some of their members think this demonstration will be the largest anti-war protest to take place in Britain while a war is actually going on.
Around 3,500 police officers have been called in to help control the London demo.
Around a million people joined a massive anti-war protest in February. It was the biggest demonstration in British history.
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