LONDON — Hundreds of thousands of protesters marched through London on Saturday afternoon in a last-ditch, long-shot effort to reverse Britain’s looming split from the European Union, calling on lawmakers to heed the enormous anger among pro-Europeans and break the political stalemate by holding a second referendum.

The protesters, some of whom traveled for hours on buses and trains, set off from Hyde Park holding placards that nodded at their European roots — “50 percent French, 50 percent British, 100 percent European,” one boy’s sign read — and employed a bit of British understatement — “Brexit really not going well, is it?” read another.

The crowd — organizers estimated a million people turned out, though there was no way to independently confirm it — clogged vast stretches of central London, with thousands of people still waiting to begin marching by the time those at the front of the rally were filling Parliament Square.