AT LEAST 15 people have been killed in a road crash in Bulgaria involving a bus carrying tourists.

Police say the bus was transporting tourists on a weekend trip to a nearby resort when it overturned and then plunged down a side road 20m below the main route it had been travelling on.

Another 27 people were injured in the incident.

The accident happened at 5.10pm local time on Saturday, 20km north of the capital Sofia.

Health officials said some of those injured were in a critical condition. There was no immediate information about the nationality of the passengers. Following the crash, the government immediately declared Monday a national day of mourning.