Police pushed back the protesters and Giri's guards safely removed him from the car.
Giri was unhurt, but his vehicle was damaged, Sarvendra Nath Khanal, Katmandu police chief, said.
Nepal Oil corporation, the state-owned oil import and distribution monopoly, raised oil product prices last week, saying it was necessary to reduce its losses and increase supplies.
The pump price of petrol rose by 25 per cent to $5.70 per gallon, while diesel went up 25 per cent to $3.80 a gallon.
Nepal Oil said it had recently been losing up to $39 million per month due to the rapid rise in global oil prices.
The company predicted the retail price increases would reduce its monthly losses to about $25 million.