MAGLAJ, Bosnia (AP) -- Packed into buses, boats and helicopters, carrying nothing but a handful of belongings, tens of thousands fled their homes Saturday in Bosnia and Serbia to escape the worst flooding in a century.
Rapidly rising rivers surged into homes, sometimes reaching up to the second floors, sending people climbing to rooftops for rescue.
Hundreds were also evacuated in Croatia.
Authorities said 20 people have died but warned the death toll could rise. Tens of thousands of homes were left without electricity or drinking water.
Landslides triggered by the floods also raised the risk of injury or death from land mines left over from Bosnia's 1992-95 war. The landslides swept away many of the carefully placed warning signs around the minefields.
Three months' worth of rain has fallen on the region in three days this week, creating the worst floods since records began 120 years ago.
Observed from the air, almost a third of Bosnia, mostly its northeast corner, resembled a huge muddy lake, with houses, roads and rail lines submerged. Admir Malagic, a spokesman for Bosnia's Security Ministry, said about a million people - over a quarter of the country's population - live in the affected area.
"Bosnia is facing a horrible catastrophe," said Bakir Izetbegovic, the chairman of the Bosnian three-man presidency. "We are still not fully aware of actual dimensions of the catastrophe ... we will have to take care of hundreds, thousands of people ..."
Izetbegovic was touring Maglaj, hard hit by floods. As the waters mostly withdrew on Saturday, Maglaj was covered in mud and debris, with residents checking damage and bringing furniture out in the streets to dry.
"Everything is destroyed, but we are happy to be alive," said Maglaj resident Zijad Omerovic.
In the eastern Bosnian town of Bijeljina, some 10,000 people were being evacuated Saturday after the rain-swollen Sava River pushed through flood defenses, endangering four villages outside the town. The peak of the Sava flood wave was expected in Bijeljina later Saturday, before advancing to Serbia.
"We need everything, we are underwater," mayor Mico Micic exclaimed.
In eastern Croatia, the overflowing Sava spread over villages and farm land, sending hundreds fleeing.
The rain caused nearly 300 landslides in Bosnia, burying dozens of houses and cars and further complicating relief efforts.
"They come unannounced in just a few seconds," said Fahrudin Solak, a Civil Protection official.
Officials in Bosnia say 12 people died and more bodies could be found as water recedes from dozens of cities. In some places, people had to be rescued by helicopter from their roofs.
Many in Bosnia lost homes they had only just rebuilt after the war, which claimed 100,000 lives and devastated the impoverished country.
In Serbia, eight deaths were reported and emergency crews and soldiers were using boats and helicopters to rescue thousands trapped in the town of Obrenovac, near Belgrade. Authorities also ordered residents of another nearby small town, Baric, to leave immediately Saturday afternoon. Many hurriedly climbed into buses and military trucks to get away.
Officials said more than 15,000 people have been evacuated from flood-hit regions in Serbia, many finding shelter in schools and sports halls. Lines of mattresses covered the floors of Belgrade schools, with frightened survivors describing unstoppable torrents that surged in a matter of minutes.
"It's all gone, all that we worked for, our animals, everything," one sobbing woman told Studio B television.
The flooding in Obrenovac is threatening the Nikola Tesla power plant, Serbia's biggest. Plant capacity had already been cut after a nearby coal mine was flooded and authorities urged residents to save energy to avoid brown-outs.
Prime Minister Aleksandar Vucic told a press conference that new wave of flooding on the Sava River will hit Sunday evening.
"Our primary concern is to protect the power plant," said Vucic. "We are doing all we can."
Thousands of volunteers responded to government's appeal to build up flood defenses along the Sava. Bused in from all over the country, the volunteers worked around the clock, stacking up sandbag barricades with soldiers and emergency crews. The town of Sremska Mitrovica was a particular concern.
"You can feel the solidarity everywhere. People are doing everything they can. I am seeing children and older people shoveling sand and carrying sandbags and no one finds this labor hard," said volunteer Marinko Trivunovic.
"I'm proud of the fact that I'm from Belgrade and I've come here to help these people to save their homes. By saving this city, we're saving the whole country," said volunteer Nemanja Radovic.
International help poured into the two nations. A Russian team joined the rescue efforts in Serbia. Rescue teams from Luxembourg, Slovenia and Croatia were already in Bosnia, and others from the U.K., Austria and Macedonia were expected.
Residents in both countries mobilized through Facebook or other social media, collecting tons of food, blankets and clothing for the crisis-hit areas.
From the Italian Open in Rome, Serbia's best tennis player, Novak Djokovic, appealed for flood volunteers on his Twitter account.
"Support for everyone! Let's help the endangered! Join the aid action!" he tweeted.