Hungarians are setting aside their differences in a race against time to defend their historic capital Budapest and other areas from what the prime minister predicted will be the "worst floods of all time".

As water levels broke records in the west ahead of an expected peak on Monday, thousands of volunteers joined soldiers and emergency workers -- often crossing traditional social boundaries to make some unlikely alliances -- to roll up their sleeves and don rubber boots.

Students, boy scouts and sports clubs helped to make up and install more than two million sandbags stacked at 16 high-risk locations along the 760 kilometres (475 miles) of the raging Danube River rushing out of Germany and Austria on its way to the Black Sea.

"Hungarians come together at a time of emergency," said one grateful local as she watched workers heave sandbags onto a truck to be taken to a barrier in picturesque, 1.7-million-strong Budapest.

Around 3,000 volunteers, 6,000 emergency service personnel, and the entire Hungarian army have been mobilised across the country. More than 16,000 army, police and civil reservists were also on standby.

The socialists and the right wing have called a temporary truce, and in the northern Romai district of the capital, Roma and the Hungarian National Guard came together in an improbable partnership to fill sandbags.

"If the water is threatening to take your house away no one cares if the person helping you is brown or white," said Gabor Veres, sporting a bright yellow safety vest emblazoned with "Hungarian National Guard".

Normally the organisation, successor to the banned paramilitary Hungarian Guard and close to the openly anti-Semitic Jobbik party, is better known for staging intimidating marches through villages populated by the ethnic Roma minority.

Veres, 25, says one of the group's main aims is to help out at "catastrophes" like this.

"It's a media myth that we have an issue with all gypsies, it's only with the criminal ones who cause the problems," he told AFP.

The low-lying Romai area is particularly exposed as the Danube suddenly widens there after curving sharply south from the Slovakian border.

Mayor Istvan Tarlos warned Thursday that if the water rises above 8.75 metres (29 feet) -- more than double the normal depth -- and threatens to burst a barrier in Romai, 55,000 people would have to be evacuated.

-- 'I'd like to think strangers would help me' --

Istvan Szilagyi, a 35-year-old Roma who lives in a nearby village just north of the capital, said he came with some friends to help out his "Hungarian friends".

"We took time off work to do this, we'll work all day today and tomorrow if we're needed and hope that the area will be safe after Monday," he said.

On the other side of the street members of the small leftwing Democratic Coalition party formed another little group filling a row of bags.

"I've made hundreds and hundreds of them, three or four shovels of sand in each, then the others take them away and put them on top of the dyke," said Gabor Szabo, a 65-year-old pensioner who lives close by -- but safe from harm 150 metres (yards) up a steep street.

"I think Jobbik are fascists, but if my house was in danger, then I'd like to think other people, strangers, would come and help me," he said.

"Here everyone is a person, no one's interested in whether you are a Roma, a conservative-nationalist or a Jew, a homeless or a Guard member. The water is just over there, and it's coming."

The Danube was already lapping up against the nine-metre dyke at the end of the street. A second "insurance" barrier is back two streets, 200 metres inland.

So far, only around 500 people nationwide have had to leave their homes and there have been no deaths, although Prime Minister Viktor Orban said Thursday that tens of thousands of people could be evacuated in the coming days.

"It is now clear that we are facing the worst floods of all time," Orban, who spent the night at a military barracks in the deluged western city of Gyor, was quoted as saying in a statement on Friday.

"I don't think we'll have to be evacuated but I've never seen a flood like this before though. It's a little scary," said 60-year-old Szasz Ferencne, who lives a few streets away from the river in Budapest.

"It's great to see them all helping. I don't care where they're from," she added.