A massive manhunt is underway in the Austrian capital after an attack that left four civilians dead. Authorities say at least one "Islamist terrorist" was behind the shootings and that more suspects may be at large.

Hundreds of police have been deployed across Vienna to search for suspects after gunmen opened fire at multiple locations in the city, killing at least four people and injuring 17.

The shootings came as many people were out and about enjoying the last evening before a nationwide coronavirus lockdown was due to come into force.

One suspected attacker, who was armed with an assault rifle and wearing a fake suicide vest, was shot dead by police.

Interior Minister Karl Nehammer told an early morning press conference on Tuesday that investigations indicated the man was a sympathizer of the extremist group "Islamic State." He added that more perpetrators may be on the run and urged citizens to stay home if possible.

Two men and two women have been confirmed dead. Health authorities cited by Austria's APA news agency also said seven victims of the attack were in a critical, life-threatening condition in hospital.

What we know so far

• Gunfire erupted outside Vienna's main synagogue shortly before 8 p.m. local time (1900 GMT/UTC) on Monday
• Authorities said there were shootings at six different locations in the city center
• Witnesses described men firing dozens of rounds into crowds at bars and restaurants with automatic rifles
• It is unclear how many attackers were involved in the attack
• At least 1,000 officers have been deployed in the search for potential attackers
• Neighboring countries, including Germany and the Czech Republic have stepped up border checks

What we know about the gunmen

• Authorities said one attacker shot dead by police appeared to have an Islamist motive
• The deceased attacker was 20-years-old and held dual citizenship in Austria and North Macedonia
• Interior Minister Karl Nehammer said the man was convicted in April 2019 because he had tried to travel to Syria to join the extremist "Islamic State" group
• Police used explosives to blast their way into the apartment of the suspected attacker
• They have also carried out searches of other properties as part of their investigation

Austria's Kurz makes plea for unity

Austrian Chancellor Sebastian Kurz described the shootings as a "repulsive terror attack" and urged the country to unite in the wake of the killings.

In a televised address on Tuesday, he vowed that Austria would defend its democracy, fundamental rights and its liberal way of life.

"We will never allow this hatred to gain ground," Kurz said in a televised speech.

"We must be aware that this is not a conflict between Christians and Muslims or between Austrians and migrants," he added. "Our enemy - Islamist terrorism - does not only want to cause death and pain, but it wants to split our society."

How have leaders reacted?

Other world leaders have expressed shock and offered condolences following the attack. German Chancellor Angela Merkel expressed solidarity with Austria, saying: "Islamist terrorism is our common enemy."

French President Emmanuel Macron was one of the first foreign leaders to react on Monday night. France has been grappling with a series of deadly terror attacks in recent weeks, including the beheading of a school teacher in a Paris suburb and a knife attack at a church in Nice.

"We must let our enemies know what they are up against. We will not give in," he said.