BRUSSELS, March 23 (Reuters) - The European Union's free trade agreement with South American bloc Mercosur will apply on a provisional basis from May 1, the European Commission said on Monday.

The key trade elements of the accord, which has proven contentious in Europe, will apply from that date between the 27-nation European Union and the countries in Mercosur that have completed their ratification procedures before the end of March.

"Argentina, Brazil and Uruguay have already done so. Paraguay has recently ratified the agreement and is expected to send its notification soon," the Commission said in a statement.

(Reporting by Philip Blenkinsop, editing by Inti Landauro)