London is a constituency of the European Parliament. It currently elects 8 MEPs using the d'Hondt method of party-list proportional representation.
The constituency was formed as a result of the European Parliamentary Elections Act 1999, replacing a number of single-member constituencies. These were London Central, London East, London North, London North East, London North West, London South East, London South Inner, London South West, London West, and parts of London South and Surrey East.
More information, including lists of elections and MEPs, on the Wikipedia page