The impact blasted two 20-ft containers full of second-hand auto parts metres away and flung vehicles across the street.
Some of the many mechanics who work in the area were trapped under burning cars in a district of the city which is no stranger to crime and sleaze.
The scene of the explosion was a condemned building which, because of the street slope, was like the basement of Kirinyaga Road Shell Petrol Station.
The building reeked of petrol, possibly suggesting leaking underground tanks. The explosion set fire to the fuel pumps at the station, which was quickly put out.
One man died at Guru Nanak Hospital where he had been rushed for treatment for multiple injuries, according to Nairobi Provincial Police boss Anthony Kibuchi.
Ballistic experts were still combing the scene to establish its cause. There were conflicting reports with some eyewitnesses claiming the explosion was caused by a gas cylinder, while others said it was a missile.
The district has one of the biggest concentrations of illegal immigrants in the city. One apartment block was packed with Pakistani immigrants.
Police and rescue officials said 36 people were being treated at Kenyatta National Hospital, MP Shah Hospital, Nairobi Hospital, Guru Nanak and Park Road Nursing Home.
Of those injured, five were in serious condition on Sunday. (IN PICTURES: Nairobi blast)
Among those injured was Ayaan Altaf, aged just one and half months, who sustained cuts on the head from broken glass.
Several others suffered burns to the face and head, fractures and cuts from the broken glass and metal.
A doctor at Kenyatta where most of the injured were taken said most of them had sustained second degree burns and tissue injuries.
“We are expecting walk-in patients which could increase the number of the casualties,” said Dr Peter Kamau Wanyoike.
Second-hand car part dealers lost property in the blast. At least ten vehicles, including five saloon cars, were extensively damaged.
The explosion occurred near Grogan Garage on lower Kirinyaga Road between Kirinyaga Road Shell Petrol Station and Shabir Centre, a six-floor residential block.
Some of the injured were tenants of the building, which was also damaged. Glass windows were shattered in most of the nearby buildings.
Eyewitnesses, describing the power of the explosion, said it lifted shipping containers and cars in the air and slammed them onto the pavement across the street.
“I saw a sharp and fiery object in the air that landed at the exact spot where the explosion occurred. This was not an ordinary explosion.
“Someone must have thrown an object with the intention to kill,” said Mr Moses Omondi, who lives near the scene.
The blast happened minutes after 11.30am, just as traders opened their shops after coming from Sunday prayers.
Prime Minister Raila Odinga visited the scene in the company of Lands Minister James Orengo, Regional Development Minister Fred Gumo and Special Programmes Permanent Secretary Andrew Mondoh. Area MP Margaret Wanjiru also visited the scene.
Mr Odinga promised government support to the victims and the traders who had lost their property in the blast.
“We came here to assess the situation and express our sympathies with those who have sustained injuries and those who have lost property,” the PM said.
Initially, there were fears that people were trapped under the two containers but no one was found after the they were removed and debris cleared.
“This is an extremely sad incident and we plead with the people to allow experts do their work to investigate the real cause of the blast,” Mr Odinga said after being taken around the scene by Nairobi Provincial Commissioner Njoroge Ndirangu.
Mr Odinga later proceeded to Kenyatta hospital to visit the victims who were undergoing treatment. Ms Wanjiru said the immediate task was to treat the victims and help them restart their businesses.
Emergency services were called in to put out the fire, take the injured to hospital, evacuate nearby buildings and keep law and order.
The police were later joined by the Military Police and ballistic experts, while members of the National Youth Service assisted in keeping order.
Earlier, there was a scuffle between fire fighters from Nairobi City Council and youths who were assisting in rescue operations.
The youths pelted the fire fighters with stones as the latter responded by directing their powerful hoses at the youths.
This is the third incident to occur in less than a month after a similar one in early May. (READ: Tragedy as four boys are killed in bomb blast)
Five school boys died in Ol Maroroi, Ngong after an explosive device went off. The device had been left behind by the military during training.
On May 24, eight factory workers were burnt to death at Picasso Chemicals, a paint factory in Nairobi’s Kariobangi estate.
Police are yet to establish the cause the blast that killed one and injured dozens of people.
Police Spokesman Eric Kiraithe said that initial investigations had not established what caused the explosion and whether it was a result of any criminal acts.
“Experts are on the scene and we are yet to know the cause,” he said.
Investigators from the Nairobi Area Provincial Criminal Investigations office were joined by bomb experts from CID Headquarters and the Anti-Terrorism Police Unit.
The scene is to remain sealed off until experts finalise their investigations.
He said that by Sunday evening, one person had been confirmed dead, 2 admitted in the Intensive Care Unit, twelve admitted in various hospitals with serious injuries and 24 were treated in various hospitals within Nairobi and discharged.
Explosions have killed and injured many people in the recent past.
In December last year, one person was killed and 41 others injured after a blast occurred at Kampala Coach stage along River Road.