The river, fed by heavy rain and melting snow in central Europe, rose to a 111-year record high on Saturday, displacing hundreds of people across the Balkans and putting tens of thousands more at risk.
A bottleneck at Serbia's narrow Djerdap gorge, near its border with Romania, caused the river to back up and water levels to rise upstream all the way to Belgrade.
The city's mayor, Nenad Bogdanovic, said he expected the waters of the Danube and the Sava river, which converge in the city, to peak on Sunday after surpassing their record highs.
"We have reinforced barriers which will resist the wave but the question is how long the water level will remain so high. That's what's worrying," said Srdjan Jovanovic, head of the Belgrade flood defense team.
He appealed to citizens to avoid a popular recreation spot on the Sava, saying some young women had pierced sandbags with their high heels, increasing the danger of collapse.
"The danger is big, and water can break the top of the dikes. The whole recreation area would be flooded in only a few seconds," he said.
Workers also scrambled to stem flooding downstream in the center of Smederovo, where waters have inundated the city's ancient fortress, train station, and other buildings.
"There are too few volunteers, unfortunately. It seems citizens don't understand seriousness of the situation and what consequences a breakthrough of the dykes might have," Smederovo mayor Sasa Radosavljevic told daily Politika.
Much of the area is still reeling from devastating floods last year which drowned scores of people and destroyed houses, farmland and infrastructure worth hundreds of millions of euros.
Downstream, the intentional flooding of a vast swath of forest and farmland in Romania prevented the river from engulfing towns there and across the river in Bulgaria.
But the move forced 113 people from their homes in the village of Rast in southwest Romania, raising the total number evacuated in that country to 750. Officials were also preparing to move 500 more in the affected zone.
"The controlled flooding is bearing fruit, triggering lower water levels in some locations," deputy Environment Minister Lucia Varga told Reuters by phone from the scene.
"We are continuously monitoring the situation downstream because we expect higher water levels in the southeast."
On the opposite bank in Nikopol, a Bulgarian town of 4,000 that was largely under water on Saturday, authorities said water levels had fallen, but soldiers and divers continued to fortify dykes there and in other partially swamped towns.
In Vidin, home to 50,000, authorities erected a tent city in case they needed to evacuate people from low-lying areas.
They said that although the river should recede slightly on Monday and Tuesday, they were not out of danger, as the flood now in Serbia was expected to reach the area and push waters higher again in the middle of the week.
"I expect a drop tomorrow. But it will not last for long and I expect another rise from Wednesday or Thursday," Hristian Kirilov, head of the Vidin civil defense office, told Reuters.
