A construction worker drinks water while working on a building during hot weather in Pristina, Kosovo, on Thursday. Reuters

Montenegro's energy minister said the shutdown had been caused a sudden increase in power consumption brought on by high temperatures, and by the heat itself. Power distribution systems are linked across the Balkans to allow transfers and trading.

"This was just waiting to happen in this heat," Gentiana, a 24-year-old student in Montenegro's capital Podgorica, told Reuters.

Electricity and wifi networks went down from around 1pm, officials and social media users said. Operators said they had started restoring supply by mid-afternoon.

Traffic ground to a halt in Bosnia's capital Sarajevo and the cities of Banja Luka and Mostar, Reuters reporters said, as temperatures hit 40˚C in parts of the region.

Many lost water in Podgorica as pumps stopped working, locals reported. Air conditioners shut down and ice cream melted in tourist shops.

There was also traffic gridlock in the Croatian coastal city of Split, state TV HRT reported. Ambulance sirens rang out cross the city, it added.

"The failure occurred as a result of a heavy load on the network, a sudden increase in power consumption due to high temperature and the high temperatures themselves," Montenegro's energy minister, Sasa Mujovic, said in a TV broadcast.

Experts were still trying to identify where the malfunction originated, he added.

Albanian Energy Minister Belinda Balluku said there had been a breakdown in an interconnector between Albania and Greece and he had heard there had been similar circumstances in Montenegro and parts of Croatia and Bosnia.

A full investigation would take time, but early analysis suggested that "big volumes of power in the transmission system at the moment and very high temperatures in record levels have created this technical problem,” Balluku added in a video address.

Power in Albania was restored within half an hour, but the country remained at a high risk of further shutdowns as power usage and heat levels were still high, he said.

Croatia's state news agency, HINA, cited unnamed sources saying the failure had started in Montenegro.

Danko Blazevic, a director with the Croatia's HOPS electro transmission company, said it was too early to say for sure what happened.

"European agencies are in charge of reporting on such outages. It would be irresponsible to speculate because the investigation usually lasts six months," Blazevic said.

Reuters