Calin Georgescu, an ultranationalist candidate who won the first round of Romania’s abruptly aborted presidential election last year, has been barred from competing in a do-over vote scheduled for May, sparking a small but violent protest by his supporters in Bucharest, the Romanian capital.

The Central Electoral Bureau issued a statement late Sunday saying that it had ruled against registering the candidacy of Mr. Georgescu, an outspoken critic of Ukraine and NATO who has voiced sympathy for Russia and Romania’s fascist leadership during World War II. The bureau also said it had rejected three other would-be candidates.

It gave no explanation for the decision, which came less than two weeks after Romanian prosecutors opened a criminal case against Mr. Georgescu for “incitement to actions against the constitutional order,” the “communication of false information” and involvement in the establishment of an organization “with a fascist, racist or xenophobic character.”

Several hundred angry protesters gathered Sunday evening outside the election bureau in Bucharest, screaming “thieves” and “traitors,” and hurling stones and firecrackers at police officers, who responded with volleys of tear gas.

The protest was far smaller than previous street demonstrations by Mr. Georgescu’s supporters but it raised political tensions and fears of violence ahead of the country’s second attempt at a presidential election. The crowd later dispersed.

The Romanian president has limited powers but has often played an important role in the foreign policy of the NATO-member country, which borders Ukraine and has a large air base near the Black Sea that is used by the U.S. military.

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