Spain's government has mobilised more than 23,000 police officers to crack down on a lorry drivers' strike against rising fuel prices.
A number of drivers have been blamed for attacking other colleagues who have not supported the walkout.
Rocks have allegedly been thrown at lorries on Spanish roads, while cargo tarps have reportedly been torn and tires punctured.
Pickets threw burning tires onto a highway in northwestern Spain overnight, leading to two arrests.
Transport Minister Raquel Sánchez has claimed that a group of far-right extremists is "bent on blackmailing this country".
"This violent behaviour is not representative of the transport sector," Sánchez said. "We are aware [of the difficulties over fuel prices], but we won't give in to this blackmail."
Police across Spain were ordered to ensure that the delivery of essential goods and services is maintained, the Interior Ministry said.
In a statement on Thursday, lorry drivers denied far-right links and accused the Spanish government of "trying to criminalise and place ideological labels" on them.
"[We are] a sector that only wants to live from its work and which feels excluded and disrespected by the country’s leaders."
The open-ended strike, which began Monday, has not been supported by Spain's main transport trade unions nor its road haulage federations.
Lorry drivers have complained that the rising cost of diesel fuel and energy and left them in a "catastrophic" situation.
The government says those involved in the strike are a minority of the country's lorry drivers.
The walkout has threatened to disrupt national supply chains, with some businesses reporting shortages of fresh produce such as milk and fish.
The Spanish government says it plans to introduce measures against high energy and fuel prices later this month.
France has also seen scattered protests this week against soaring fuel prices. Farmers, road hauliers and fishing crews have set up temporary barricades, using their vehicles and burning palettes to block roads.