Spanish lawmakers on Thursday passed a building reform that promises to protect the country's coasts, but environmental groups complained it would not save the miles of shoreline already blighted by concrete.

The lower house of parliament controlled by the ruling conservative Popular Party (PP) passed the bill which pledges to protect the "sensitive and vulnerable" coastline -- but also the owners of properties already built close to the shore.

Environmental campaigners warned it would merely entrench private building projects in coastal zones where many unsightly concrete blocks blossomed under previous legislation dating to the 1980s.

As lawmakers debated the bill, Greenpeace activists scaled the lower house of parliament in Madrid and hung a banner reading "the PP is selling our coasts", until police climbed up and chased them down.

Greenpeace said police arrested eight people involved in the protest.

Among the contested clauses in the law is one that extends leases of built-up coastal land to 75 years.

Environment Minister Miguel Arias Canete said that was to protect buildings built before 1988 from being demolished when leases expire.

It also reduces to 20 metres in some cases the protection zone that must be kept between buildings and the sea.

Greenpeace's coastal affairs spokeswoman Pilar Marcos said the law "sets us back 25 years in environmental protection" and panders to the "private interests" of property developers.

"It is the culmination of the barbarities of the building boom," she said in a statement.

The environmental spokesman for the opposition Socialists in parliament, Jose Luis Abalos, said the reform "puts an end to the most serious attempt ever made in Spain to protect our coasts".

He said that only 30 percent of Spain's coasts were not occupied by human activity.

The government insisted the reform would protect Spain from a repeat of infamous projects such as the Algarrobico, a 21-story concrete hotel built in a national park on the southern coast.