Vienna police said on Wednesday that it had increased patrols due to a general threat of attack against churches.
Austria's public security directorate had received indications that an attack with an "Islamist motive" had been planned in Vienna, police said.
"You will notice there are currently a larger number of police forces on patrol with special equipment," Vienna's police force tweeted.
"The reason is a non-specific threat of attack against churches," it said.
Police said that this represented a common "preventative measure."
Vienna's police force urged residents not to share images or videos of officers or to spread rumors about operations.
Police said that it could not yet be established how long the enhanced security measures would be in place.
"Police officers are equipped with bulletproof helmets and vests and assault rifles. They will carry out surveillance activities and also carry out checks in road traffic," police spokesman Markus Dittrich told the Radio Wien station.
Catholic churches stay open
Michael Prüller, a spokesman for the archdiocese of Vienna, told the Associated Press (AP) that Catholic churches did not appear to be the main target.
"We do not appear to be primarily affected," the spokesman said. "While we were informed by police about the general threat, we were also told that there is no imminent danger for Catholics.
The archdiocese has "decided to keep (its) churches open to the public and celebrate all church services as planned for the time being," he said.
AP reported that St Stephen's Cathedral, located in downtown Vienna, was packed with believers and tourists on Wednesday.
Local media reported that the Syrian Christian community could be the main target of the attack.