Fuel protests triggered by rising oil prices have spread to more countries across Europe, with thousands of fishermen on strike.
In Spain, Europe's largest fish producer, the action is expected to bring the industry to a halt.
Fishermen in France have been protesting for weeks, and their counterparts in Portugal, Belgium and Italy are also joining the campaign.
UK and Dutch lorry-drivers held similar protests earlier this week.
The strike reflects anger at the rising cost of fuel, with oil prices above $130 (83.40 euros; £65.80) a barrel.
Trade unions say the cost of diesel has become prohibitively high, after rising 300% over the past five years.
Wholesale fish prices, meanwhile, have been static for 20 years.
The European Commission has promised immediate help to restructure Europe's fishing industry, but it says subsidies to offset rising fuel costs would be illegal.
Several thousand fishermen are expected to march on the agriculture ministry in Madrid, where they have promised to hand out 20 tonnes of fresh fish to members of the public in an attempt to draw attention to their ailing industry.
Spanish trade unions also say they could blockade ports, a day after French police forcibly removed fishermen blocking oil depots.
"We must mobilise like the French and if we have to block ports, we'll block them," Xavier Aboy, a union leader in the north-western Galicia region, told AFP news agency.
In France the authorities have offered 100m euros in aid, prompting some fishermen to return to work.
At dawn on Thursday, French riot police cleared protesters from the Mediterranean oil depots of Fos-sur-Mer and Lavera, and a Total refinery at La Mede in the south.
On the same day police clashed with protesters who burned tyres in the Atlantic port of Lorient, while hundreds protested in Quimper, Brittany.
Hundreds of farmers have also been blocking oil terminals near the cities of Dijon and Toulouse.
In Italy, at least 5,000 fishermen are expected to strike, the main trade union Federcoopesca says. The government has already refused emergency aid to the industry.
Bulgarian bus drivers are also planning a one-hour strike on Friday, following protests by lorry-drivers on Wednesday.