Seventy-three members, known as MEPs, will be elected in nine constituencies in England, and one each in Scotland, Wales and Northern Ireland.
Each region has a different number of representatives based on its population – ranging from three MEPs in the North East and Northern Ireland to 10 MEPs in the South East.
Polling stations in the UK are open until 22:00 BST.
How the BBC reports polling day
European elections: What you need to know
The Netherlands is also voting on Thursday while voting in other EU nations will take place at various times over the next three days.
The results will be announced once all EU nations have voted, with the voting process expected to be completed by 22:00 BST on Sunday.