A powerful storm with strong winds and heavy rain hit Croatia and Slovenia on Wednesday, killing at least four people and injuring several others, police and local media outlets said.

The storm was the second in two days to sweep over the two countries following a string of extremely hot and dry days. Elsewhere in Europe, a heat wave caused wildfires and public health warnings.

Two men died in Croatia's capital, Zagreb, after they were hit by falling trees, Croatian police said. A 50-year-old man was struck while outside in the street, and a 48-year-old man was in his car, a police statement said. Earlier, emergency doctor Ljupka Hitrova told the official Hina news agency that the two victims had multiple head injuries.

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The civil protection service in eastern Croatia reported that one person died when a tree fell on their car in the town of Cernik.

Elsewhere in Zagreb, a 36-year-old man was severely injured when a construction crane collapsed, the police statement said.

The storm suddenly darkened the sky and brought blinding rain, flash floods and winds across Zagreb starting at about 4 p.m. It halted the city's transportation, caused power outages and left streets blocked with fallen trees.

Europe Fox News graphic

Slovenia and Croatia have reported at least four deaths caused by violent storms in the aftermath of a heatwave. (Fox News)

Zagreb’s emergency services asked citizens to be patient as hundreds of calls for help came in at once.

"We are appealing to the citizens to call only if in a life-threatening situation or of utmost necessity until the situation stabilizes," Zagreb Mayor Tomislav Tomasevic said on Twitter.

A similar situation was reported in neighboring Slovenia, where high wind toppled trees and blew off roofs.

Public broadcaster RTV Slovenia reported that one person was killed and another was injured when a tree fell on them in the Lake Bled area.

Authorities in Serbia issued a warning to citizens as the storm moved east Wednesday. Wind gusts suddenly swooshed through the capital Belgrade on Wednesday evening, swirling leaves and dust as people scuttered for protection.

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Powerful storms also caused chaos in Slovenia, Croatia and Bosnia late Tuesday, toppling trees, tearing roofs off buildings and causing power outages.

Emergency services in the three countries reported hundreds of interventions as the storm swept through the region. It also brought a much-sought relief from the heat.

Slovenian authorities said tens of thousands of people were left without electricity and scores of campers evacuated their camps as strong winds overturned their tents on Tuesday night.

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The greatest damage was inflicted on high-altitude forests, the official STA news agency reported. Authorities warned climbers in the Alpine country to stay away from those areas.