Officials and resources were urgently mobilised to help people in areas of Sukhothai municipality which were abruptly hit by the flooding.
The surging waters made many roads in municipal areas of Sukhothai's Muang district impassable, with the level more than a metre deep in some places. At least five schools suspended classes.
His Majesty the King has had disaster relief packages worth more than 760,000 baht, delivered to 1,500 flood-affected households in the province via the Rajaprajanugroh Foundation.
The government said water management at dams was still under control, and that the Sukhothai flood was due to the levee bases on which flood walls had been built breaking unexpectedly.
Sukhothai municipality's Wichian Chamnong, Ratchathani, Pracha Ruamjai, Khlong Tapetch, Phra Maeya and Tan Diew communities were inundated.
Local residents were forced to evacuate their belongings to higher ground.
Government offices, including the Sukhothai provincial hall, the provincial prosecution office and the Muang municipality office, were also swamped.
As floodwaters rose, the Provincial Electricity Authority decided to temporarily cut the water supply in several areas.
Workers from the municipality, local administrative bodies and a highway agency in Sukhothai toiled around the clock yesterday to put up large sandbags at locations where floodwater entered. Water pumps were expected to be turned on to drain out water from downtown areas after the floodwater is blocked.
Soldiers and border patrol police were mobilised to help local residents evacuate their belongings to safe areas and a rescue centre has been set up at Wat Ratchathani to help flood victims.
Nisarat Khlibo, 50, a vendor at a municipality-run market, said many vendors were caught off guard by the sudden flooding. By the time a flood warning was issued, she saw water gushing into the market. Her grocery shop was under water less than an hour after the warning was issued.
Royol Chitradon, a member of the Water and Flood Management Commission (WFMC), said the status of water management in major dams was in line with the commission's plan.
He said water level in the Yom River was still lower than the floodwalls in Sukhothai by almost one metre.
Floodwater was able to enter inner Sukhothai because lower parts of the levees were eroded by the flow of water.
Prime Minister Yingluck Shinawatra, who chaired the WFMC meeting yesterday, ordered the provincial governor and authorities to provide urgent assistance to Sukhothai residents, he said.
Ms Yingluck will visit the North on Thursday to inspect the flood situation.
Lertviroj Kowitwattana, director-general of the Royal Irrigation Department, said the North's major dams have the capacity to handle expected rainfall.
But if there is significant rainfall south of the Bhumibol Dam in Tak and Sirikit Dam in Uttaradit, several rivers may overflow, he said.
Over the past week, it rained heavily in several areas. In Phitsanulok, flood run-off has already reached Bang Rakam district, inundating more than 4,000 rai of farmland, a source said.
Thawit Luesingbun, head of Tabaek-ngarm village, said heavy downpours coupled with a huge volume of northern run-off caused Khlong Ket and Khlong Kam canals to overflow, submerging paddy fields in the village.
Luckily, he said, most farmers have already completed harvesting their crops.
In Ayutthaya, downpours and run-off badly damaged thousands of rai of paddy fields in Pak Hai district. The Noi River yesterday burst its banks, flooding riverside communities in Pak Hai with water levels reaching 30-50cm.
In Suphan Buri, three districts were under water after the Tha Chin River burst its banks. Days of heavy rain caused the river to overflow and inundate Muang, Bang Pla Ma and Song Phi Nong districts.