"We fear the death toll may rise because the condition of some of the injured is very critical," said Aftab Cheema, regional police chief in Faisalabad, the country's textile-making capital.
"It was a car bomb blast. The explosive was planted in a car. We are investigating whether a suicide bomber was involved or not," Cheema said.
Television pictures showed the station had been reduced to a pile of bricks and gnarled metal as rescue officials worked to remove rubble from the scene to search for survivors and ambulance vehicles ferried the injured away.
City commissioner Tahir Husain told private Geo television that rescue officials were heaving bricks and metal away to save those trapped.
"There are some people trapped under the building rubble. We have deployed our cranes and machinery to rescue them very soon," he added.
Husain told private TV channel Waqt that no suicide attacker was involved.
"It was not a suicide attack. It was a planted bomb blast. The bomb exploded near the gas cylinders that triggered a bigger blast," he said.
Husain said that the attack could have targeted government buildings close to the filling station, which sold compressed natural gas for vehicles, adding that some of the buildings were damaged in the blast.
Pakistan has been wracked by violence, mostly targeting security officials.
Some 4,000 people have been killed in bomb blasts, suicide and gun attacks blamed on Taliban and Al-Qaeda fighters since Pakistan troops stormed a militant mosque in Islamabad in July 2007.
On Saturday at least one person was killed and another wounded when a bomb exploded inside a house in Pakistan's biggest city of Karachi.
Faisalabad city is near the home of a Christian government minister who was buried Friday after being killed in a hail of bullets in Islamabad last week over his opposition to the country's strict Islamic blasphemy laws.
His assassination sparked international outrage and stoked concern about rampant militancy and rising extremism throughout the nuclear-armed nation, a fractious ally in the US-led war in Afghanistan.