"Calin Georgescu was about to submit his new candidacy for the presidency," his team said in a post on Facebook.
"He was taken in for questioning at the General Prosecutor's Office."
It is not yet clear why prosecutors have taken the 62-year-old Georgescu in for questioning or whether he will be charged with anything.
No details were immediately available.
The Constitutional Court made the unprecedented move to annul the presidential election just two days before a December 8 run-off.
Georgescu - who was polling in single digits and declared zero campaign spending - surprised many when he won the first round on November 24, after which allegations of electoral violations and Russian interference emerged.
Thousands of protesters took to the streets of Bucharest on Saturday in a show of support for Georgescu, who has repeatedly denied any wrongdoing and argued the election was "cancelled illegally and unconstitutionally".
New dates have been set to rerun the vote with the first round scheduled for May 4.
If no candidate obtains more than 50 per cent of the ballot, a run-off would be held two weeks later on May 18.
Georgescu remains voters' top choice in opinion polls ahead of the May re-run although it remains unclear whether he will be allowed to run.
Dozens of house searches of Georgescu supporters have also been carried out across the country, the public prosecutor's office in Bucharest said.
The searches were related to campaign financing and suspected illegal right-wing extremist propaganda.
Members of US President Donald Trump's administration have sharply criticised Romania for annulling its election.
US Vice President JD Vance said the cancellation meant Romania does not share US values while billionaire Elon Musk labelled the chief judge of the top court that annulled the vote a "tyrant".
with Reuters and DPA
