Firefighters recovered the body of an elderly man who drowned in his flooded basement apartment, and emergency services were still searching for another aged person missing near Ostia, a town on the coast near Rome.

As the lightning and thunder persisted, both metro lines were heavily disrupted, with stations remaining open to provide shelter to commuters and tourists caught in the downpour.

There were severe delays on trains and flights out of Rome's Fiumicino airport. Cars with flooded engines blocked traffic in the capital's cobbled streets around the President's palace and the Sapienza University.

Rome's mayor Gianni Alemanno said he would declare "a state of emergency" but reassured citizens that the rain should ease up in the afternoon, significantly reducing the risk that the city's Tiber river would burst its banks.

Bang Bua Thong, only a few kilometres (miles) outside Bangkok in Nonthaburi Province, had for weeks been spared the worst of the disaster, thanks to the district's hastily assembled defences.

But a mass of water from further north fuelled by months of unusually heavy monsoon rains proved too much -- and by Thursday, the district was submerged.

"It was very scary when the water came. It came up so fast through drains, toilets and the floor tiles under my cabinet. I have no idea how it came up through the floor tiles," Ruchuda Balisee, 40, told AFP.

"Roads near my house are destroyed."

An exodus was seen with people wading from their homes with bags, boxes and suitcases, while others searched for isolated relatives facing shortages of food, water and electricity.

Rescuers prioritised the safety of children and the elderly, a number of whom were greatly distressed after several days in isolation.

In front of the hospital, fire trucks were immersed up to the top of their wheels.

The street became a vast expanse of murky water, on which all kinds of boats moved in silence, instead of the usual flow of cars and motorcycles.

The fear was palpable. Many Thais are unable to swim but people had no choice but to wade through chest-deep waters. Some residents settled on bridges to await rescue.

At the hospital, patients were heard complaining about the lack of boats to rescue them, but others in the district were more accepting of the situation.

"I am fairly satisfied with the government's support," said Chat Thongthammachat, 62, waiting for a boat. "I know that many people need help so I do not blame them."

It is a scene that residents of Bangkok fear will be repeated in the capital, which has so far been protected by beefed up flood defences.

Prime Minister Yingluck Shinawatra warned Thursday it was impossible to stop the floods gushing into the city and ordered sluice gates to be opened to allow water to flow through canals and out to sea.

"The longer we block the water the higher it gets," she said.

The floods have killed 320 people around the country, damaged the homes and livelihoods of millions of people -- mostly in the north and centre -- and forced tens of thousands to seek refuge in shelters.

The prospect of serious flooding sparked a new round of stockpiling of food and water, while police warned residents to stop parking their vehicles on the elevated highways around the city.

"It is both dangerous and inconvenient," said General Panu Kerdlapphol, adding that many vehicles had already been towed away by the authorities.

Many homes and businesses in the capital have piled sandbags outside their entrances and soldiers have been deployed to protect floodwalls as anger grows among residents upriver bearing the brunt of the crisis.

Adding to the fearful mood, the fisheries department said a special "rapid movement" team has been set up to catch crocodiles which escaped from flooded farms, mostly in central Thailand.

"If you encounter a crocodile, don't panic," it advised in a statement. "Normally they are afraid of human beings. So just use a wooden stick or your hand to bang the water and the crocodile will swim away."