In a move that polarized voters, Romania's top court voided the presidential election on Dec. 6, two days before the second round, Reuters reported.
The cancellation came after state documents showed frontrunner Calin Georgescu, a critic of NATO, had benefited from an unfair social media campaign likely to have been orchestrated by Russia, accusations Moscow has denied.
The court ordered that the election be re-run in its entirety. The pro-European coalition government has yet to approve a calendar for the election, although party leaders agreed to hold the two rounds on May 4 and May 18.
Iohannis, whose term expired on Dec. 21, will stay on until his successor is elected.
On Sunday, tens of thousands of protesters, including left-wingers and those angered by the way the way the election was cancelled, joined the protest organized by the opposition hard-right Alliance for Uniting Romanians (AUR), Romania's second-largest party.
"We ask for a return to democracy by resuming the election with the second round," AUR leader George Simion told reporters.
Organizers said 100,000 people were at the protest, but riot police along the march estimated the numbers at around 20,000. Protesters waved flags and shouted "Freedom" and "Bring back the second round."
"Our right to vote was broken," said Bogdan Danila, a 43-year-old truck driver. "In addition, Iohannis was in power for ten years and did nothing for the people, while parties betrayed us, they are all corrupt. We want something else."
Some protesters carried portraits of Georgescu or Christian Orthodox icons while street vendors sold flags and vuvuzelas.
"Authorities must say why they cancelled the election, we want to see the evidence," said Cornelia, 57, an economist wrapped in a Romanian flag who declined to give her last name.
"At this rate we won't be voting anymore, they will impose a leader like in the old days."
It remains unclear whether Georgescu, who opposes Romanian support for Ukraine against Russia, will be allowed to run for president again.