The Babel club in Malmo, Sweden, had its entire entrance blown apart in the early hours of Friday morning.
A witness tweeted: “Nightclub has just been bombed in Swedish city of Malmö. Several loud explosions heard on the scene.”
Cops and a bomb squad descended on the scene with the area placed on lockdown for hours.
Another bystander said: “I was awakened by a big crowd. It was a terrible bang.”
A Malmo Police officer said in a statement: “At the moment, we do not know what was detonating, just because it caused extensive material damage.”
The explosions rang out at around 12.30am (11.30pm UK time) and were so powerful they could be heard across the city.
Cops are said to be urgently seeking witnesses after people reported seeing a moped speed away from the scene around the time of the blast.
This is the second time the club has been targeted, with a similar bomb attack taking place in October 2016.
Early reports circulating on social media had claimed revellers fled for their lives, but the club’s owner said it was open earlier in the evening but had closed by the time the explosions occurred.
No injuries were reported.
Malmo is home to a huge migrant population, which has been blamed for a spate of incidents.