Thousands have marched through London to oppose the renewal of Britain's Trident nuclear weapons system in what demonstrators describe as the biggest such rally in a generation.

Waving placards and banners, the protesters offered their opposition Saturday ahead of a final government decision on whether to replace the current generation of submarines that carry the nuclear warheads.

Britain's Ministry of Defense says getting the new submarines will cost 31 billion pounds ($43 billion) over a 20-year procurement program, together with another 10 billion pounds for unexpected cost increases. Supporters believe Trident is critical to national security, but Labour's long-held support is now in doubt, as leader Jeremy Corbyn is opposed to such weapons.

Scottish National Party leader Nicola Sturgeon told the crowd that Trident was both "immoral" and "impractical."