BUDAPEST, MARCH 20 -- An attack on ethnic Hungarians Monday night by an ax- and club-wielding mob in Romania has triggered an outraged official reaction in this country and prompted a mass nationalist demonstration in Budapest.

The attack in the Transylvanian town of Tirgu Mures, which Hungarian officials say left at least two dead and several others severely injured, is the most serious ethnic clash in Romania since the overthrow last December of dictator Nicolae Ceausescu, whose government brutally repressed the rights of ethnic Hungarians.

Hungarian Foreign Minister Gyulya Horn today demanded immediate action from the United Nations and the Romanian government to prevent more violence, while Prime Minister Miklos Nemeth sent a personal note to Romanian Premier Petre Roman. In his messages, Horn expressed "profound anxiety . . . over the serious atrocities committed against ethnic Hungarians in Romania over the past few days."

At dawn today, the Romanian army sent reinforcements to Tirgu Mures to protect the local headquarters of the Democratic Union of Hungarians, a newly formed ethnic organization that was the focus of Monday's attack, the Hungarian news agency MTI reported.

{The Reuter news agency reported new violence in Tirgu Mures late Tuesday, as about 2,000 Romanians armed with scythes and other farm tools attacked a crowd of ethnic Hungarians gathered to protest the Monday attack. Officials said one person was killed and scores injured.}

Here in Budapest, Istvan Csurka, an outspoken nationalist writer and founding member of the Hungarian Democratic Forum political party, told a crowd of about 10,000 angry, flag-waving protesters in Heroes Square that the attack "shows that dictatorship has not been diminished totally. Romania must chose now which way to take. Ceausescu is dead but his spirit seems alive."

Hungary is in the final stages of its first multi-party election campaign since 1945, and four political parties sent well-known speakers to the square in an apparent bid to score last-minute points with voters.

Monday's attack in Transylvania comes at a time when a far-right nationalist movement called Vatra Romaneasca appears to be gaining strength in Romania. Hungarians here say the group has been joined by remnants of Ceausescu's Securitate secret police and is playing on the resentments of Romanians toward the 2.2 million ethnic Hungarians who live in the region. Transylvania was part of Hungary before 1920, when the Treaty of Versailles assigned it to Romania.

Budapest's relations with Romania had been strained for years by Ceausescu's systematic mistreatment of ethnic Hungarians, and the government had hoped, Horn said, that the dictator's death would usher in a new era of friendly ties between the two countries.

News agency reports of Monday's attack, based on interviews with ethnic Hungarians in Tirgu Mures, said that several hundred Romanians from villages around the town arrived in a convoy of buses and cars and besieged the offices of the Democratic Hungarian Union. The organization, which was formed after the death of Ceausescu, is pressing for the right of ethnic Hungarians to practice their culture and language.

Reuter reported that four members of the organization, including author and playwright Suto Andras, 61, were rescued from the building by police but that the mob later stormed a police van they were in and assaulted them. "We shall fight, we shall die, but we shall not give up Transylvania," the Romanians were quoted as shouting.

A report Monday in the British newspaper Financial Times quoted a leader of Vatra Romaneasca, Octavian Capatina, as saying that Hungarians want to "regain all the privileges they had after the Second World War" before Ceausescu took them away. He accused the new Bucharest government of being "too democratic, too European, not Romanian."

Romania's provisional president, Ion Iliescu, appealed on television Monday for calm and apologized for "the regrettable excesses . . . of some Romanian citizens."