Brown has an advantage, for in a British election that produces no clear winner political convention allows the incumbent the first attempt to form a government. He has more than a week to hold talks aimed at reaching a pact with smaller parties.
Anger flared when voters in London, Sheffield, Newcastle and elsewhere complained that they had been blocked from voting as stations closed - and the head of Britain's Electoral Commission said some legal challenges to results because of blocked votes were likely.
Police had to quell a sit-in protest in east London by 50 angry residents who were denied the chance to vote. Liz Veitch in Hackney said she'd been frozen out after waiting for more than an hour and a half, with a line still stretching down the street.
"There are an awful lot of extremely angry people around here," she said. "It's an absolute scandal."
Crowds tried to block officials from taking the ballot boxes in Sheffield, as officials struggled to cope with staggering turnout.
Electoral Commission chief Jenny Watson acknowledged that Britain's paper voting system had been unable to cope with a surge of voters.