LONDON — Tens of thousands of demonstrators descended on the British capital on Saturday in a noisy but peaceful protest at a government austerity drive aimed at slashing the nation’s debt.

Unions, antiwar campaigners, left-wing leaders, community groups, and other activists poured down London’s streets in a demonstration against reductions to public sector spending which officials are pushing through in order to rein in the Britain’s debt, which stands at more than $1.7 trillion.