Europe's disaster commissioner told Bulgaria on Wednesday that it needs to boost its preparations to deal with emergencies after a 5.8-magnitude earthquake triggered panic among sleeping residents.

"Luckily, there were no casualties but people were very scared, which tells us that we have got work to do," Crisis Response Commissioner Kristalina Georgieva, who is Bulgarian, told state BNR radio in an interview.

"It is not the first time that we have seen that people do not know exactly what do do in such cases ... We need to be better prepared."

Although the quake in the early hours of Tuesday did not kill anyone, terrified residents rushed out of their apartment buildings in Sofia still clad in their pyjamas. The epicentre was less than 30 kilometres (20 miles) southwest of the capital.

Rather than go home, many residents huddled in their cars overnight or stayed on the streets, despite the falling drizzle.

Few remembered to take their identity cards or a radio -- the only source of information at this early hour -- and many admitted their reactions were inadequate.

The first official information about the magnitude of the quake was also not forthcoming for more than an hour after it struck, prompting people to flock to social networks in a bid to get information.

A 62-year-old woman from Kyustendil (45 kilometres southwest of the epicentre) died from a heart attack overnight Tuesday, which was possibly caused by shock after the quake, said the Presa newspaper. The authorities have not confirmed so far any casualties from the quake.

"Don't leave the buildings during a quake!" Trud newspaper also warned Wednesday above a full-page list of "Dos and Don'ts" released by the civil defence.

"We rely solely on the professionals -- firefighters and rescuers," Georgieva commented, slamming the lack of any volunteer-based disaster response units in Bulgaria.

The quake-prone country also needs to boost disaster prevention and improve ordinary people's preparedness and reactions, she added, suggesting special lessons in kindergartens and schools.