Protesters on Wednesday flooded Madrid in a massive show of force against biting austerity policies, capping a Europe-wide day of strikes and protests.

Activists, chanting, whistling and setting off firecrackers, swamped a stretch of about two kilometres (more than a mile) of a broad avenue from the main Atocha railway station to the symbolic Plaza de Colon square.

Many tens of thousands of people joined the rally in the Spanish capital, swarming around the main squares, where red-and-white union banners were strung up declaring: "They are taking away our future."

Others gathered around the lower house of parliament, which was sealed off by riot police who deployed in large numbers and were backed by a police helicopter.

Scores of people began to assemble before police barriers set up on Neptuno Square to block access to the nearby parliament building. A dozen police vans were parked behind the barrier.

Protesters faced off with police, decrying the policies of Prime Minister Mariano Rajoy's government, which defends austerity measures as the only way to curb a yawning public deficit and exit the crisis.

They brandished banners reading "Mariano go home!", "Crisis? Robbery!" and "Spain evicted!", a reference to a surge in home-owner evictions blamed for two recent suicides.

On Friday, 53-year-old former Socialist politician Amaia Egana jumped out of her apartment window to her death in the northern Basque Country as bailiffs prepared to evict her.

"We have the solution, send the bankers to prison!" protesters chanted in a sea of flags of the main CCOO and UGT unions, who organized Spain's second general strike in just eight months.

Many of the demonstrators, who also turned out in large numbers in other parts of Spain including in northeastern Barcelona, wore T-shirts declaring their allegiance to education and public health.

"Public health for all," said some shirts.

Dotted around all the protests were signs displaying giant scissors overlayed with the word "No".