Elections are taking place for the Scottish parliament, National Assembly of Wales, the Northern Ireland assembly and for 124 councils in England, BBC reported.
New mayors will be elected in London, Bristol, Liverpool and Salford, with parliamentary by-elections in Ogmore and Sheffield Brightside.
Voters in the 40 police force areas in England and Wales, excluding London and Greater Manchester, will also elect a police and crime commissioner. The inaugural elections for the positions were held in 2012.
Polling stations across the country opened at 7.00 a.m., with voting allowed until 10.00 p.m.
Counting will take place throughout Friday with the majority of results declared on the day.
However, some English council results will not be announced until Saturday while the final result of the Northern Irish election is not expected until Sunday.