The Bangkok authorities on Monday asked people in six Bangkok districts to evacuate, as the water run-off from the north started to seep towards the inner areas.
The evacuation order was issued as areas along the Chao Phraya River experienced high tide.
The six districts are Don Mueang, Laksi, Bang Phlat, Sai Mai, Thawee Watthana and Taling Chan.
Residents in 13 districts bordering the Chao Phraya River were also on alert Monday for another day of high tides.
Residents in seven additional districts - Bang Khen, Lat Phrao, Wang Thong Lang, Khan Na Yao, Nong Chok, Min Buri and Lat Krabang - must be vigilant and monitor the incoming northern run-off.
Meanwhile Prime Minister Yingluck Shinawatra remains confident that the floodwater will not reach the inner part of the city, saying some parts of the capital were seeing more water than others due to breached dykes, which are being repaired.
Latest reports say the northern run-off is flowing toward the inner city reaching Laksi intersection on Vibhavadi Road. On Phaholyothin Road, it is nearing Bang Khen roundabout while floodwater in front of Kasetsart University is flowing slowly to Kasetsart intersection.
On Chaeng Wattana Road, floodwater is rising as Klong Prem Prachakorn and Klong Prapa overflow into nearby residential areas.
More northern run-off is flowing into Klong Prem Prachakorn, posing a flood threat which could extend to the Lat Phrao intersection via Vibhavadi Road