The Harmony of the Seas, the world's largest cruise ship, set off on Thursday on its first sea trial from Saint-Nazaire in western France, with just two months to go before delivery.

The city's STX France shipyards began building the 1 billion euro ($1.1 billion) mammoth for US cruise ship line Royal Caribbean International in September 2013.

Thousands of people gathered at the dock to watch as the 120,000-ton ship was helped out to sea by six tugs.