Austrian police said Monday that several people were injured and officers were out in force following gunfire in the capital, Vienna.
Initial reports that a synagogue was the target of an attack couldn't immediately be confirmed.
Austrian news agency APA quoted the country's Interior Ministry as saying that one attacker has been killed and another could be on the run.
Police said the operation in the center of the city was ongoing and urged people to avoid all open spaces and public transport.
"There are several injured persons," police tweeted. "We are on site with all available forces. Please avoid all public squares in the city."
Police added that the exact circumstances of the incident were still being determined.
Austrian public broadcaster ORF cited witnesses as saying several shots were fired shortly after 8 p.m. (1900 GMT).