The collision near the capital Sofia follows an upsurge in violent incidents at Bulgaria's borders in recent months.
Five people were injured when a minibus carrying Syrian migrants rammed into a police checkpoint, Bulgarian officials have said.
The vehicle -- carrying 12 suspected migrants -- failed to stop at a checkpoint near Sofia on Tuesday and later crashed into a police car.
A 51-year-old policeman suffered life-threatening head injuries in the incident, the Bulgarian interior ministry said. A 20-year-old migrant also sustained a head injury, while three other passengers on board the minibus were hospitalised.
Police say the Bulgarian driver of the vehicle is known to them and was driving without a licence.
The incident comes just months after two Bulgarian police officers were killed when they were hit by a bus carrying migrants.
Earlier this month, a 30-year-old policeman was also killed by gunfire while patrolling near the border with Turkey.
Bulgaria's interior ministry has called for migrant smugglers to face tougher sentences than fines amid an increase in migrant crossings.
Since January, Bulgaria has arrested 14,427 illegal immigrants, compared to 8,254 in the whole of 2021, according to the interior ministry.
"We are witnessing an epidemic of this kind of crime," said Marian Marinov, the investigating judge in charge of the case.