The flooding hit Gwaelod-y-Garth in the Taff’s Well area following an explosion at a nearby quarry owned by the Breedon Group.
Surface water inundated streets and properties from around 2pm, forcing residents to leave their homes as water poured onto public roads and residential areas.
A spokesman for South Wales Fire and Rescue Service said: “We are currently responding to surface water flooding at Heol Berry and Heol Y Nant, Gwaelod-y-Garth in the Taff’s Well area.
“Please avoid the area to allow the emergency services to deal with the incident.”
Multiple fire crews and a rescue tender from Whitchurch, Pontypridd, Ely, Roath, Caerphilly, and Penarth attended the scene, working to evacuate residents and minimise flood damage.
South Wales Police were also at the scene managing traffic and public safety. Officers closed roads at the junction of Heol Goch and Main Road and urged people to avoid the area.
There are reports that an underground lake beneath the hill may have emptied during the blast, causing the flood.
Pictures from the scene showed water pouring from the doors of evacuated homes on affected streets.
Residents told of homes destroyed by what some described as a “wall of water” following the incident.
The flooding also led to the evacuation of a nearby industrial estate.
Natural Resources Wales (NRW) confirmed they were investigating the environmental impact on the River Taff, which showed signs of discolouration.
A spokesman for NRW said: “Our thoughts are with those affected by today’s incident at Gwaelod-y-Garth.
“We are aware of the visible discolouration of the River Taff resulting from this incident.
“Our officers are on site to assess the environmental impacts and are working closely with the emergency services to advise on minimising any further risk of pollution to watercourses in the area.
“We are aware of significant damage to a public right of way through the woodland as a result of the incident and urge people to avoid the area.”
On social media, residents said clean-up operations had begun and sandbags were being distributed.
The incident remains ongoing as of 9pm on Thursday, July 30.
