VALLETTA, Malta—The tiny Mediterranean country of Malta has sworn in as premier the 39-year-old leader of the Labor Party, who captured the biggest electoral victory since independence.

Joseph Muscat was cheered by huge crowds of supporters Monday as he became the second-youngest premier in Malta's history.

His party won 55 percent of the vote in Saturday's national election to oust the Nationalist Party after 15 years in power which garnered 43 percent of the vote. It was the largest victory by a political party since the archipelago obtained independence from Britain in 1964.

The Labor Party is expected to have a nine-seat majority in Parliament, a contrast from the Nationalist Party's one-seat majority in the last legislature.

Muscat's first duty is to appoint ministers before an EU summit Thursday.