The Commonwealth Heads of Government Meeting (Chogm) will be officially opened by the Queen in Malta.
Security is high at the Mediterranean Conference Centre as media and government leaders from around the world arrived at the venue.
David Cameron and Foreign Secretary Philip Hammond are both in Valletta for the summit, which is held every two years.
The Prime Minister has announced plans to set up a Commonwealth unit targeting the extremist "scourge" that is fuelling international terrorism.
He has pledged £5 million to help countries find new ways of stopping "poisonous ideologies" from taking hold.
The Queen will give a speech to the delegates, presidents and prime ministers from the 53-nation group, which is the start of two days of meetings with the leaders.
The event is part of the Queen's three-day royal tour of the island, accompanied by the Duke of Edinburgh, the Prince of Wales and the Duchess of Cornwall.
There will also be an opening performance which will feature more than 360 artists and performers, including the Maltese Philharmonic Orchestra and tenor Joseph Calleja.
After the event the Queen and Charles will hold a reception for new Commonwealth heads of government.
This evening the royals will attend a Chogm banquet at the Corinthia Palace Hotel in Valletta.