Chattogram has recorded 412mm of rain in 24 hours, the highest rainfall in the city in 42 years.
The Bangladesh Meteorological Department’s data covers the 24 hours from 3pm on Monday to 3pm on Tuesday.
The last time the port city saw such rain was on Aug 4, 1983, when 511mm was recorded over a 24-hour period.
High tide has worsened flooding in the area, submerging more neighbourhoods.
Water has inundated at least three roads, while flooded railway tracks left the Parjatak Express train stranded.
Several key roads were submerged, including the stretch of Port Connecting Road from Halishahar Housing Estate to Naya Bazar, the Baro Dighir Par section of the Oxygen-Hathazari Road, and parts of Arakan Road near C&B and Maulvi Pukur Par.
Waterlogging was also reported in Katalganj Housing Estate, the Agrabad commercial district, CDA Residential Area, Panchlaish, Kapasgola, Chawkbazar, Rampur, Tin Poler Matha, Batali Road, the Custom House compound, Jamalkhan Bylane, Rahmatganj, Halishahar Housing Estate, Agrabad Beparipara Kitchen Market, Maulvipara and Kathgor Muslimabad.
Most of the city's canals overflowed, sending water onto surrounding roads. Authorities opened sluice gates built under a canal drainage project around midday to help ease flooding.
Assistant Meteorologist Suman Saha of the Patenga Weather Office confirmed the figures to bdnews24.com, saying the downpour was the city's second-highest 24-hour rainfall on record.
Rain began falling across the port city on Sunday morning. It measured 40mm by 6pm that day before rising to 206mm in the following 24 hours ending at 6pm on Monday.
Exceptionally heavy rain intensified on Tuesday morning, coinciding with high tide that triggered widespread flooding.
The tide began around 11:45am, preventing accumulated rainwater from draining through canals into the Karnaphuli River and the Bay of Bengal.
Floodwaters steadily rose across Agrabad, Katalganj, Chawkbazar and other neighbourhoods, inundating the ground floors of many homes in Katalganj.
Waist-deep water submerged the Baro Dighi Par area of South Pahartali on the Oxygen-Hathazari Road, where residents were seen casting fishing nets on the flooded main road.
On Arakan Road, vehicles of all sizes pushed through floodwater near C&B and Maulvi Pukur Par, while heavy vehicles navigated submerged sections of Port Connecting Road between Halishahar and Naya Bazar.
The torrential rain also washed away part of a bypass road in Patenga. At about 12:40pm, floodwater covering the railway tracks at Sholoshahar left the Cox's Bazar-bound Tourist Express stranded with around 650 passengers on board.
The skies remained overcast well into the afternoon, and shortly after 3:30pm, another spell of torrential rain swept across the port city.
FA/R