The turnout has been relatively low, with just some 23 percent of the country's 18 million voters going to polling stations by mid-afternoon.
Romania's parliament suspended Basescu on April 20, accusing him of violating the constitution, despite a Constitutional Court ruling that the charges were groundless.
Basescu says his opponents initiated the impeachment process to stop his pro-reform and anti-corruption drive.
Basescu has been locked in a long-running, bitter power struggle with liberal Prime Minister Calin Popescu Tariceanu, a former ally of the president.
Voter opinion polls have suggested that the president, who remains popular with the public, will easily win the vote and remain in office.
On May 18, Foreign Minister Adrian Cioroianu warned that the patience of the European Union, which Romania joined in January, "has a limit," and that after the referendum politicians should stop fighting and "get back to work."
(compiled from agency reports)