A wave of anti-austerity anger is sweeping across Europe with general strikes in Spain and Portugal and walkouts in Greece and Italy - grounding flights, closing schools and shutting down transport.
Millions of workers have been taking part in the dozens of co-ordinated protests in a so-called European Day of Action and Solidarity against spending cuts and tax hikes.
In Portugal, violent clashes took place between activists and police officers in Lisbon, and one woman was seen being repeatedly struck with a baton.
In Italy, local media reported that six police officers were injured, including one seriously, as clashes broke out with protesters at demonstrations in Milan and Turin.
Around 20 activists allegedly beat an officer with a stick and baseball bats in Turin, while five officers were hurt during running street battles in central Milan.
In Spain - the fourth-biggest eurozone economy - activists and unions will be staging an evening rally outside the parliament in the Madrid.
Protests got under way early in Madrid and Barcelona, with protesters clashing with police as they attempted to blockade buses and cause disruption at food markets.
Riot police fired rubber bullets at hundreds of protesters in Madrid's Cibeles Square and used night-sticks to quell picket line disturbances, as more than 80 people were arrested.
Airlines operating in the country including Iberia, Iberia Express, Air Nostrum, Vueling, Air Europa and easyJet cut more than 600 flights including some 250 international routes.
Heathrow said, of its normal 1,300 daily services, 39 flights - nearly all involving Spain or Portugal - had been cancelled.
Hospitals in Spain fully staffed emergency and surgery rooms, but non-essential care was scaled back.
Spain, where one in four workers is unemployed, is now teetering on the brink of calling for a European bailout, with Prime Minister Mariano Rajoy trying to put off a rescue that could require even more EU-mandated budget cuts.
Italy's biggest trade union CGIL called strikes in dozens of cities - and thousands of workers took to the streets calling for more safeguards for jobs and pensions and protesting against Prime Minister Mario Monti's government.
Student Mario Nobile, 23, said: "Europe is waking up today - from Rome to Madrid to Athens."
Protests are also being called in 40 towns and cities across bailed-out Portugal, including Lisbon and Porto.
Portuguese airline TAP said it was grounding more than 160 flights, most of them international.
Greece, struggling to satisfy international lenders that it has cut spending sufficiently to qualify for bailout funds and to avoid default, has called a three-hour walkout and a rally in Athens.
The European Trade Union Confederation said it was the first time that it had appealed for a day of action that includes simultaneous strike action in four countries.
"By sowing austerity, we are reaping recession, rising poverty and social anxiety," its general secretary Bernadette Segol said in an online statement.
"In some countries, people's exasperation is reaching a peak. We need urgent solutions to get the economy back on track, not stifle it with austerity. Europe's leaders are wrong not to listen to the anger of the people who are taking to the streets."
Union-led rallies are also being called across France and in Poland, while high-speed Thalys rail services between Belgium and Germany have been cancelled for the day.
Just 20% of Spain's long-distance trains and a third of its commuter trains are expected to run, while Lisbon's Metro will be shut completely with only 10% of rail services in action.
Tensions have been rising in Spain since last Friday when a woman jumped from her apartment to her death as bailiffs tried to evict her from her home in the country's second apparent suicide linked to evictions.
On Monday, the country's largest banks agreed to halt repossessions for the most vulnerable for two years.