Floodwaters from Thailand's flood-ravaged central heartland pushed farther into Bangkok on Monday, as residents of long-submerged provinces north of the capital started to rebuild their lives.

The water slowly advancing through Bangkok's northern and western neighborhoods is threatening the city's subway system, two key industrial estates and the emergency headquarters set up to deal with the flooding that has claimed more than 500 lives nationwide.

Evacuations have been ordered in 12 of Bangkok's 50 districts, with residents of the northern district of Klong Sam Wa told to leave Monday. The evacuations, which also effect parts of several other districts, are not mandatory, and many people are staying to protect homes and businesses. But the orders illustrate how far flooding has progressed into the city and how powerless the government has been to stop it.

Residents row a boat in front of a bus along a flooded street in Bangkok on Monday. Chaiwat Subprasom

The flooding began in late July and some provinces to the north of Bangkok have been inundated for more than a month. The waters have started to recede in recent days, revealing the massive cleanup effort that lies ahead.

For two months, Anan Dirath was forced to live on the second floor of his home in Nakorn Sawan province. But now that the water has receded to knee level, it's time to clean up.

He armed his two teenage children with mops, scrub brushes and garbage bags. Wading in the water, his family began scrubbing dirt off the walls and collecting the garbage around the house. He said the dirt was difficult to wash off and he has had to scrub the paint off to get rid of it.

"Oh my pretty home. It used to be a pretty two-storey home," he said Monday.

In nearby Nakorn Sawan town center, where the water has dried completely, the government sponsored a cleanup day last week when roads were scrubbed down to get rid of the oily mud left from the floods. Backhoes were used to carry garbage away.

The cleanup also has begun in some parts of Thailand's ancient capital of Ayutthaya. Prime Minister Yingluck Shinawatra is to visit the province Tuesday to witness recovery efforts.

Yingluck says a plan to be put before the Cabinet on Tuesday would allocate 100 billion baht ($3.3 billion) for post-flood reconstruction.

Her government has come under fire for failing to predict the threat to Bangkok. Residents also have been frustrated by widely differing assessments of the flooding situation from the prime minister, Bangkok's governor and the country's top water experts and officials.

Floodwaters in the city continued to flow south Monday toward the still-unaffected central business district. In Chatuchak, a few kilometers north of there, water was nearly knee deep around Mo Chit Skytrain station, the northernmost stop on the capital's elevated train system.

Water was also rising near three subway stops in the same area. Both mass transit networks are functioning normally, though some exits have been barricaded and closed.

Chatuchak is home to the government's national flood relief headquarters, which is housed in the Energy Ministry — a building now surrounded by water. The relief headquarters moved last week out of Bangkok's Don Muang airport after it, too, was flooded. The city's main airport remains open.

Also in Chatuchak, water has begun approaching a main road near the Mo Chit bus terminal, a major gateway to northern Thailand.