Around 2,800 localised flooding incidents have been reported to the Flood Line in the Belfast, Antrim, Bangor, Dundonald and Lisburn areas after thundery showers swept across the region on Wednesday evening.
Further heavy rain is forecast for the early hours of the morning. The Met Office has issued a new amber weather warning valid from 4am to 9pm on Thursday.
"Following the downpours of Wednesday evening, the public should be prepared for further flooding to occur," the Met Office says.
Police say most of the main arterial routes in east and south Belfast are impassable, including Annadale Embankment, the Knock Dual Carriageway, Knocknagoney Roundabout, Belmont Road, Upper Lisburn Road and Finaghy areas.
"The roads in these areas are impassable as a result of flooding, abandoned cars and debris," a spokeswoman said.
Police have urged drivers to exercise extreme caution across the city.
"Police are advising motorists not to travel unless it is absolutely necessary. Motorists who have abandoned cars are asked not to return to their vehicles, for their own safety, at present," the spokeswoman added.
"Police are currently receiving a high level of calls which are being prioritised. Officers are working with all the relevant statutory agencies involved."
In east Belfast, the Castlereagh Road, Ladas Drive and King's Road have also been affected.
The Dundonald Ice Bowl has closed and water has also seeped into part of the Ulster Hospital.
Meanwhile the Ormeau Road, Upper Malone area, Lisburn Road and Finaghy are some of the worst hit areas in the south of the city.
The Stockman's Lane onslip at the M1 west bound was also closed for a time but has now re-opened.
Fire crews have also been pumping water out of Main Street in Cushendall, Co Antrim.
At this time we would appeal to the public to assist us during this period by avoiding any contacting NIFRS on the 999 service unless there is an imminent life risk and use the flood line 0300 2000100 to report flooding incidents
NI Fire & Rescue Service
The Fire and Rescue Service said it also been inundated with calls.
The NIFRS said: "Northern Ireland Fire & Rescue Service (NIFRS) is experiencing a large volume of operational activity as a result of a large number of flooding incidents.
"Since 18:30 hours this evening NIFRS has responded to numerous calls, many involving requests for further assistance from local crews to deal with flood water.
"While our resources are heavily committed we can assure the public that additional resources have been made available to support our operational response."
NI Water says all incidents so far have been localised flooding from drainage systems which are unable to cope with the volume of rainfall.
"Every effort has been made to alleviate the flooding in the affected areas as quickly as possible and assist with the clean-up operation," they said.
Meanwhile around 1,000 properties in the Balmoral, Newforge and Milltown areas suffered power cuts, NIE confirmed.
"Approximately 1000 homes in the South Belfast area have been restored to power following a fault on the electricity network in the area earlier this evening," a spokeswoman said.
"NIE emergency crews are investigating the cause of the fault. Due to the flooding around Belfast, Northern Ireland Electricity has opened its Local Incident Centre in Belfast this evening to respond to any disruption to supplies."
John Wylie of the Met Office told UTV he expects the wet weather to continue into Thursday morning.
Areas of thundery rain will move north to affect Northern Ireland, with the potential for slow moving torrential downpours to develop through the afternoon and lasting into the evening.
"There have been hourly rainfall totals of an inch or more in places this evening which have really overwhelmed some drains and led to flash floods," Mr Wylie said.
"This will not be the end of it - there is more heavy rain to come this evening and into Thursday morning and there is a risk of further flash flooding in places."
Anyone affected by the flooding should contact Floodline on 0300 2000 100.
Any customer affected by out-of-sewer flooding should contact NI Water on 08457 44 00 88.
Areas affected in Belfast include (Source: DOE):
- Stockman's Lane closed under Flyover.
- Knock Road, closed between Castlereagh Road and Shandon Park.
- Upper Knockbreda Road / Cregagh Road. Road closed.
- Finaghy Road North closed at M1 Bridge.
- Hillhall Road. Reported as closed.
- Stewartstown Road / Twinbrook Road. Reported as closed.
- Castlereagh Road / Loopland Park. Road closed.
- Milltown Road / Belvoir Road. Passable with care.
- Stockman's Lane under M1 Bridge. Passable with care.
- Blacks Road under M1 Bridge. Passable with care.
- Milltown Road / Belvoir Road. Passable with care.
- Prince William Road / Lisburn North Feeder Road. Passable with care.