Governor Talal Barazi said the attack near the Al-Ahli hospital, in the Al-Zahraa neighbourhood where most residents belong to the same Alawite sect as President Bashar al-Assad, killed 16 people.
Another 54 were wounded, some seriously, and being treated at hospitals throughout the city.
The vehicle used in the attack had been carrying at least 150 kilos (330 pounds) of explosives, Barazi added.
In its statement, the jihadist IS named the bomber as Abu Ahmed al-Homsi, and said he parked the car and detonated it before then activating his explosives belt.
IS said the toll from the attack was more than 25 dead, with 70 wounded.
Firefighters, security forces and dozens of other men packed into the residential street where the blast occurred, searching for victims and seeking to rescue the injured.
The Syrian Observatory for Human Rights also said 16 people were killed and dozens wounded, adding that some were in critical condition.
The attack comes just three days after hundreds of rebel fighters and civilians were evacuated from the city following a deal with the regime.
The deal, overseen by the United Nations, will see Homs' last rebel-held neighbourhood, Waer, turned over to government forces in exchange for humanitarian aid for besieged residents.
- 'Body parts on ground' -
"The explosion was terrifying. Body parts were on the ground... It was one of the biggest explosions to hit Homs," a 28-year-old woman working in a neighbourhood coffee shop told AFP.
"The blast left a huge crater, and people were running in every direction in fear of more explosions," she added.
Syria's government condemned the attack, calling it a "cowardly terrorist explosion".
"These terrorist explosions will not discourage the Syrian people from national reconciliation and fighting terrorism," said Prime Minister Wael al-Halqi in comments carried by state news agency SANA.
Once known as the capital of Syria's revolution, Homs will be under the full control of the regime by the end of January.
The Britain-based Observatory and local activist groups said a UN convoy carrying humanitarian aid had been due to enter Waer on Saturday, but there was no confirmation from UN representatives in Damascus.
More than 250,000 people have been killed in Syria's conflict since it erupted in March 2011 with anti-government protests.
Elsewhere on Saturday, 10 people, including a child, were killed in air strikes on the rebel-held town of Atareb in the northern Aleppo province, the Observatory said.
It was unclear whether the strikes were conducted by government warplanes or Russia's air force, which has been carrying out an air war in Syria since September.
The monitor relies on a network of sources on the ground to collect its information, and identifies aircraft based on their flight patterns and munitions used.
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Another 54 were wounded, some seriously, and being treated at hospitals throughout the city.
The vehicle used in the attack had been carrying at least 150 kilos (330 pounds) of explosives, Barazi added.
In its statement, the jihadist IS named the bomber as Abu Ahmed al-Homsi, and said he parked the car and detonated it before then activating his explosives belt.
IS said the toll from the attack was more than 25 dead, with 70 wounded.
Firefighters, security forces and dozens of other men packed into the residential street where the blast occurred, searching for victims and seeking to rescue the injured.
The Syrian Observatory for Human Rights also said 16 people were killed and dozens wounded, adding that some were in critical condition.
The attack comes just three days after hundreds of rebel fighters and civilians were evacuated from the city following a deal with the regime.
The deal, overseen by the United Nations, will see Homs' last rebel-held neighbourhood, Waer, turned over to government forces in exchange for humanitarian aid for besieged residents.
- 'Body parts on ground' -
"The explosion was terrifying. Body parts were on the ground... It was one of the biggest explosions to hit Homs," a 28-year-old woman working in a neighbourhood coffee shop told AFP.
"The blast left a huge crater, and people were running in every direction in fear of more explosions," she added.
Syria's government condemned the attack, calling it a "cowardly terrorist explosion".
"These terrorist explosions will not discourage the Syrian people from national reconciliation and fighting terrorism," said Prime Minister Wael al-Halqi in comments carried by state news agency SANA.
Once known as the capital of Syria's revolution, Homs will be under the full control of the regime by the end of January.
The Britain-based Observatory and local activist groups said a UN convoy carrying humanitarian aid had been due to enter Waer on Saturday, but there was no confirmation from UN representatives in Damascus.
More than 250,000 people have been killed in Syria's conflict since it erupted in March 2011 with anti-government protests.
Elsewhere on Saturday, 10 people, including a child, were killed in air strikes on the rebel-held town of Atareb in the northern Aleppo province, the Observatory said.
It was unclear whether the strikes were conducted by government warplanes or Russia's air force, which has been carrying out an air war in Syria since September.
The monitor relies on a network of sources on the ground to collect its information, and identifies aircraft based on their flight patterns and munitions used.
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