Kyrees Sullivan, 16, and 15-year-old Harvey Evans were hit by a vehicle close to their homes in the Ely area at around 6pm on Monday.
Violence broke out soon after when rumours circulated on social media claiming the best mates were being chased by police – something denied by senior officers.
They said the collision had already occurred by the time police arrived, with officers then remaining at the scene to manage the ‘large-scale disorder’ which carried on into the early hours.
Bridy Bool, who lives nearby, said she knew Harvey’s family and his mother wanted to know which hospital her son had been taken to.
‘The police are not telling her where her boy is – she has been waiting all night and morning,’ she said.
‘They are not telling them – they are waiting for the family liaison officer. It was 2.40am when her boy’s body got moved from there. I’ve just left her house now.
‘There’s no support. They are on the phone but there is no support on the ground – all this is going on and they are nowhere.’
Ms Bool claimed the fatal crash occurred because ‘they were being chased by the police’.
‘I know they were being chased by the police as there are videos going around that the police were chasing them,’ she said.
‘They didn’t have helmets on and there was no reason for them to be chased.
‘Harvey had just eaten his tea and had left his mum’s. He had been at home all day with his mum, and he went with his friend, and this happened.’
Paying tribute to the teenager, she said: ‘Harvey was such a young boy, a sociable boy, he had loads of friends and he loved motorbikes and football.
‘He was best friends with Kyrees and were into the same things. It was not unusual for them to be together.’
South Wales Police said arrests have been made but did not say how many.
Specially trained public order officers were deployed, including officers from neighbouring police forces, as several vehicles were set alight, property was damaged and officers were injured.
Scenes being livestreamed on YouTube showed young people throwing fireworks and other missiles at a line of police officers with riot shields who were blocking one end of the street.
Shortly before midnight a car was set on fire and burned fiercely, while a second vehicle was overturned and also torched.
Jane Palmer, the owner of a burnt-out Ford Focus on Highmead Road, said she and her family watched from their window as rioters set fire to her car.
Ms Palmer said: ‘I’m disabled so now I’m trapped without my car. Why are they doing this? It’s just silly now.’
A man called Connor, who also lives in Highmead Road, said: ‘It’s been a mad night. This is not the norm around here but people are pretty anti-police and that’s reflected on both sides, not that that’s my opinion.’
One person was attacked because rioters thought they were an undercover officer, according to a senior officer at the scene.
Assistant Chief Constable Mark Travis said: ‘First and foremost our thoughts are with families of the two boys who have died following the collision in Ely and with those affected by the disorder which followed.
‘These are scenes we do not expect to see in our communities, particularly a close-knit community such as Ely.
‘The level of violence towards emergency services and the damage to property and vehicles was totally unacceptable.’
The riot was condemned by various politicians from across the political spectrum.
The Prime Minister’s official spokesman said: ‘The reports of disorder, violence, arson and specifically attacks on police officers are appalling and unacceptable.
‘We remain grateful to the emergency services and first responders for all their support to the community and restoring order.’
Alun Michael, South Wales Police and Crime Commissioner, told BBC Radio 4’s Today programme: ‘It would appear that there were rumours, and those rumours became rife, of a police chase, which wasn’t the case.
‘And I think it illustrates the speed with which rumours can run around with the activity that goes on on social media nowadays, and that events can get out of hand.’
Heledd Fychan, who represents the South Wales Central region for Plaid Cymru, said: ‘There must now be a full investigation to understand how this tragic incident led to the violence that followed to ensure peace is restored and that this never happens again.’
Cardiff Council’s leader called for calm and said it is ‘really important’ a full investigation into the crash can take place immediately.
Councillor Huw Thomas said: ‘This is a tragic event and they, and we, need to understand what happened and why.
‘The best way we can achieve this is by letting the investigation begin as soon as possible. Any further trouble will only delay that investigation.’