At least 21 students and several police were injured on Sunday, police and witnesses said.
Students were stopped as they attempted to hold rallies outside at least five colleges in Katmandu, police officials said on condition of anonymity in accordance with official policy.
Police said they fired tear gas and charged the students with batons outside two of the colleges, adding that 21 students were injured in the clashes.
They also used batons to halt the protests at the remaining three colleges.
Police officials said 18 students were detained. It was not clear whether they would be charged.
The students opposed a decision by Nepal's royal government to raise prices of petroleum products. Such measures usually result in higher food and transport costs.
On Friday, the prices of petrol, diesel, kerosene and aviation fuel were raised by five Nepalese rupees ($0.07) per litre.
The state-owned Nepal Oil Corp, which has a monopoly over imports and distribution of oil in the country, said it was forced to make the raises due to high world oil prices.
Nepal imports all of its oil products from neighbouring India.