The Austria Press Agency reported Monday that the country's top security official says several people have been injured in an attack in the capital Vienna and there may be fatalities.

APA quoted Karl Nehammer as saying that authorities believe there were several attackers involved and that it was still ongoing. It earlier cited Nehammer's office saying that one attacker was killed and another could be on the run.

Vienna police urged people to avoid all open spaces and public transport in the city. Police said trams and buses weren't stopping and urged social media users not to post videos of the ongoing police operation, so as not to endanger officers.

“There are several injured persons,” police tweeted. “We are on site with all available forces. Please avoid all public squares in the city.”

Oskar Deutsch, the head of the Jewish community in Vienna, said the shooting took place in the street where the city’s main synagogue is located but that it wasn’t clear whether the house of worship had been targeted.

The synagogue was already closed at the time of the shooting around 8 p.m. (1900 GMT), Deutsch tweeted.

Rabbi Schlomo Hofmeister told The Associated Press that he saw at least one person fire shots at people sitting outside bars in the street below his window.

“They were shooting at least 100 rounds just outside our building,” Hofmeister said.

“All these bars have tables outside. This evening is the last evening before the lockdown,” he added. “As of midnight, all bars and restaurants will be closed in Austria for the next month and a lot of people probably wanted to use that evening to be able to go out."