Story highlights Police clear anti-austerity protesters from Madrid's Puerta del Sol plaza

"The new government is the same," a woman says, claiming people are fed up

Similar rallies are held in Barcelona and other cities around Spain

The protests coincide with the anniversary of the launch of the May 15 movement

Chanting "they don't represent us," tens of thousands in Madrid railed early Sunday against Spain's government and austerity cuts -- venting their anger on the first anniversary of the so-called May 15 protest movement.

Many ignored a government deadline to disperse by Saturday night from the central Puerta del Sol plaza, prompting police to clear the square by 5 a.m. on Sunday (11 p.m. on Saturday ET), the interior ministry said.

About 30,000 attended the Madrid protest, and 18 were detained for resisting arrest or disorderly conduct, the ministry said.

In the early hours of Sunday morning, demonstrators were a loud and vibrant presence in the square -- as a large number of police, stationed at a nearby government building and along side streets, looked on and let them be.

Throngs of like-minded demonstrators also gathered over the weekend in Barcelona and about 80 other cities around Spain.

Barcelona saw about 22,000 protesters, while Valencia had 8,000 and Seville had 2,000, authorities said. All the demonstrations were cleared by Sunday morning, the interior ministry said.

The coordinated events marked the return of the "indignados" -- or the outraged, as the protesters became known -- who led Europe's first serious and significant grassroots movement against austerity and government budget cuts.

Similar demonstrations decrying governments' attempts to get their budgets in order, sometimes by slashing public funding, later emerged elsewhere around Europe.

In Madrid this weekend, marchers from the north, south, east and west descended on Puerta del Sol plaza on Saturday evening.

For hours, demonstrators shouted, jumped, sang and waved white handkerchiefs. Their most dramatic moment, though, may have been their quietest: when they held their hands aloft, silently, in a "silent shout" before erupting in cheers.

The crowd is expected to return. The government has approved three more days of protests in Madrid, meaning similar scenes could play out into the middle of the week.

The number of demonstrators in Madrid over the weekend appeared to be slightly fewer than those who had gathered in the same spot -- in what's known as ground zero of the movement -- a year earlier.

Back then, protesters encamped in Madrid and other cities made their voices heard. The tens of thousands of people who turned out in the initial days grew to an estimated 6 million protesters over the following months, in a nation of 46 million people.

Since then, Occupy camps around the world have come and gone.

The new protests organized by the May 15 movement are different in at least one key respect: a new conservative government is now in control, having taken over in December.

Spain's economic crisis also has worsened since last year. The nation has slipped back into a recession, the unemployment rate has risen to 24% overall and more than 50% for those under age 25, and the government has enacted billions of dollars in austerity cuts, along with some tax hikes, to reduce the budget deficit.

"We are really tired of this situation," said Madrid protester Paola Alvarado, a purchasing agent. "And the new government is the same. They steal our money and give it to the banks."

Spain's austerity protests have been largely peaceful to date, with only occasional clashes between protesters and police, and some arrests in cities like Barcelona and Valencia.

And prior the latest protests, the new government -- which has vowed to maintain order and prevent a repetition of encampments in Madrid and beyond -- urged police commanders to use "common sense" as to how they dealt with the latest round of public dissent.

In recent months, Spanish trade unions, traditionally the protest leaders, have been at the forefront of demonstrations against the austerity cuts and labor market reforms, with the May 15 movement barely visible.

"Maybe the most important thing is it awakened a consciousness, beyond concrete changes, to make historic change possible," said Jon Aguirre Such, who was a movement spokesman a year ago but now spends more time on his architectural cooperative for urban planning.

"I think everyone who took part in the May 15 movement made history. They can take away from us many things, but not our memory and our dream," Aguirre said.

The original May 15 movement is credited with helping stop dozens of housing evictions. Activists pressured bank and court officials to delay or stop foreclosures on delinquent mortgages.

But Ignacio Urquiza, a sociologist who has studied the movement for the left-leaning Fundacion Alternativas, said there has been little big-picture change as to government policies and operations.

"The demonstrations didn't do more than expose -- for a brief time -- some issues. But Spain's economic crisis and political system have not changed. They are the same as last year," he said.