Voting gets under way amid drizzle
Voting is under way in polls to elect councils across England, Scotland and Wales, as well as mayors for London, Liverpool and Salford.
London mayor Boris Johnson and his Labour rival Ken Livingstone were among the first to head for the polling stations under drizzly skies.
Voting is taking place across 181 local councils throughout England, Scotland and Wales. Voters in 10 cities are holding referendums on whether to have elected mayors.
Mr Cameron and Labour leader Ed Miliband were also early visitors to polling stations in London, accompanied by wives Samantha and Justine, while Lib Dem leader Nick Clegg cast his ballot alone in Sheffield, as Spanish wife Miriam does not have a vote.
While many councils will begin counts after the polls close at 10pm, helping to give a clear picture of the parties' performance, a large number have delayed until Friday.
A delayed count will also mean a wait to find out the result of the highest-profile contest as Mr Livingstone attempts to seize back the London mayoralty he lost four years ago. The final decision of Londoners is not expected to be known until Friday evening.
Rain is often blamed for contributing to low election turnouts and forecasters have predicted downpours for Wales and central and southern England. But MeteoGroup, the weather division of the Press Association, said voters in northern England and Scotland should escape the wet weather, with dry and mild conditions expected.
Meteorologist Paul Mott said: "It will be a cold and wet morning for central and southern areas of England, and much of Wales. There'll be heavy rain across the south midlands throughout the morning, with patchy rain for parts of southern England, so it will be fairly wet for anyone venturing outside.
"The rain will ease later in the day but it will still be a cold and damp afternoon. In northern England and Scotland, though, it will be dry and very mild at about 18 or 19C, with the highest temperatures in Glasgow."