Serbia was struggling to keep its power system going, officials warned Thursday, after weeks of record low temperatures in Europe that have snarled traffic, frozen rivers and challenged officials to step up outreach to the vulnerable homeless.

The Serb state power company said its system cannot hold on for much longer and authorities urged citizens to save electricity in an appeal aired on national television.

Europe's big freeze so far has claimed hundreds of lives, mostly of homeless people, while tens of thousands more remain trapped in remote villages in Bosnia and Serbia and other hard-hit areas.

In many European capitals, authorities have set up extra shelters for the homeless to help them survive the cold snap that has seen temperatures sink as low as 33 Fahrenheit (minus 36 Celsius).

In Berlin, rescue vanes loaded with food and clean sheets are picking up those in need and taking them to shelters.

"I witnessed a man laying on the street and people were passing by, not caring about him," said Wolfgang Gerhard, a pastor who works for the Berlin city mission and drives one of its two rescue vehicles.

"And that's what moves me: that someone could die on a street and nobody calls an ambulance."

The cold and snow have blocked roads, halted shipping on rivers, and forced schools to close in many countries. In Serbia, the power company warned of restrictions unless electricity consumption is reduced. The company said it can only meet the present level of demand for a week longer, and announced that heavy industry will be switched off first.

"We have hit the limit with imports too, because of the very difficult situation in the entire region," said Zoran Manasijevic, a senior official at the Serbian state electric company.

Manasijevic said the frigid temperatures have slowed digging for coal, which is used heavily in Serbia for power. There are also fears that ice that has formed on rivers could jeopardize power production at hydroelectric plants.

Massive ice floes have halted shipping on parts of the mighty Danube, Europe's main commercial waterway, which winds 2,860-kilometer (1,777-mile) from Germany to the Black Sea.

Authorities in Serbia plan to use explosives to break the ice to try to prevent flooding of the kind that hit Greece and Bulgaria on Monday and Tuesday, leaving dozens of homes under water and at least eight dead.

Elsewhere in the region, rescue services have been using helicopters to reached stranded people and deliver food to snowed-in regions.

In Berlin, Gerhard drove his bus to the Berlin Zoo train station, a gathering place for the homeless. There he met a a 46-year-old woman who would only identify herself as Katie. She acknowledged these days she is happy just to survive.

"If you are homeless and sleep on a park bench," she said, "when you wake up in the morning after two three hours of sleep you know you have made it through the night."

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Kerstin Sopke in Berlin contributed to this report.