Guy Verhofstadt, the leader of the Alliance of Liberals and Democrats for Europe (ALDE), celebrated a "historic" European election night, which he claimed will change the "balance of power".
Preliminary results signalled a strong showing for his liberal party, and he vowed to push for a "more ambitious Europe".
The Liberal and Democrat grouping were on course to win 101 seats, with the traditionally dominant centre-right and socialist blocs losing their majority in the European Parliament.
"This evening is a historical moment because there will be a new balance of power in the European Parliament," he said, addressing the European parliament in Brussels as the election results came in.
Verhofstadt again signalled his intention to form a new group within the the EU Parliament by allying his ALDE group with French President Emmanuel Macron's La Republique En Marche party, along with other "reform-driven parties".