* Worst flooding in Balkans in over a century
* Death toll in Bosnia reaches 19
* Rush to defend west Serbian town against rising water (Updates toll, adds quote, colour)
By Marko Djurica
OBRENOVAC, Serbia, May 17 (Reuters) - More than 20 people have been killed in the worst floods in more than a century in Serbia and Bosnia, authorities said on Saturday, with thousands evacuated from towns still under threat from rising rivers.
The death toll in Bosnia alone reached 19, including nine found on Saturday when waters receded from the northeastern town of Doboj.
Thousands of volunteers joined soldiers, police and fire-fighters in building flood barriers made of sandbags in the Serbian capital Belgrade and the western town of Sabac.
The River Sava hit its highest-recorded level in Serbia, the army said, rising at a rate of three centimetres (one inch) per hour after several days of the heaviest rainfall in almost 120 years.
Three people were confirmed dead in Serbia by Friday, and Prime Minister Aleksandar Vucic said there were more fatalities in the town of Obrenovac, 30 km (18 miles) southwest of Belgrade, where soldiers deployed huge amphibious vehicles to rescue hundreds of people crammed into a primary school.
Authorities in Serbia said they would not give a death toll for Obrenovac, a town of some 30,000 people, until the waters had receded and the extent of the damage was clear.
A Reuters photographer said the entire town centre was submerged under two to three metres (seven to 10 feet) of water.
Tens of thousands of homes in Serbia were cut off from electricity and around 150,000 in Bosnia, where Doboj suffered the most.
"It was especially difficult in Doboj because the flood waters acted as a tsunami, three to four metres high. No one could have resisted," said Gojko Vasic, police chief in Bosnia's autonomous Serb Republic.
In Belgrade, residents donated food, clothes and bedding. Police appealed for more boats. A steady rain fell on Saturday and more was forecast for Sunday.
WAIT AND HOPE
"Now we have to sit and wait, to wait for that next wave and to hope," Vucic told a joint news conference with Bosnian Serb leader Milorad Dodik.
In the Bosnian border town of Bijeljina, authorities said they would evacuate 10,000 people. More than 15,000 have already been evacuated in Serbia.
"We left behind the car, motorcycle, tools, all our furniture, valuables," said Dragana Ilic, an Obrenovac resident evacuated to a shelter in Belgrade. "We just grabbed our mobile phones and left. All our IDs were left behind. The whole house is under water."
In Bosnia, helicopters evacuated people from the northern towns of Samac and Modrica and trucks and bulldozers carried food to the hardest hit areas.
About 1,000 people, including babies, pregnant women, invalids and the elderly were evacuated from the region of Zeljezno Polje in central Bosnia, where hundreds of homes were destroyed in landslides.
"I think we'll never be able to return to our village," local Muslim imam Zuhdija Ridzal told Reuters by telephone from Zeljezno Polje. "It has disappeared".
A Reuters cameraman said people were still leaving the area by foot. The roofs of houses and cars poked out from under mud, trees and rocks.
"Only three houses survived, all others were buried, totally gone," said one villager, Ragib Menzilovic.
On Friday, Serbia's state-run power utility Elektroprivreda Srbije (EPS) trimmed output at its largest hydro power plant, Djerdap 1, on the Danube river by a quarter.
It also closed down 1,650 MW in capacity of its largest coal-fired power plant Nikola Tesla (TENT), on top of a 10 percent cut in total output a day before.
Flooding of the Kolubara, the Danube and the Sava rivers brought down cables and transformer stations, soaked coal depots that feed the power plant and caused a fire inside the Kolubara complex which had been shuttered since Thursday. (Additional reporting by Daria Sito-Sucic and Maja Zuvela in Sarajevo, Fedja Grulovic in Belgrade; Writing by Matt Robinson; Editing by Sophie Hares)