Prime Minister Jose Luis Rodriguez Zapatero's government says the amnesty will allow it to manage migration. However, its assurances have done nothing to quell the controversy.

Spain is the main gateway to Europe for illegal immigrants. An estimated 800,000 migrants live in the country illegally, but there is no way to know how many will benefit from the amnesty.

The scale of the problem was underlined on Saturday when Spanish authorities rescued 227 African would-be migrants from a decrepit fishing boat drifting off the Canary Islands.

Jose Manuel Soria, head of the local authority in the island of Gran Canaria, wrote in the conservative ABC newspaper on Sunday that the incident showed the drawing power of the amnesty offer - even though new arrivals cannot qualify.

"The irresponsible announcement of papers for everyone redoubles the risk and the ingenuity of the clandestine expeditions," said Soria, a member of the opposition Popular Party which has roundly criticized the amnesty.

El Mundo newspaper said in an opinion piece that it was no coincidence that Spain was facing an influx of immigrants from Africa and of east European migrants through the Pyrenees in the days before the amnesty took effect.

The amnesty is an attempt to recognize that thousands of migrants are already working illegally without paying taxes or joining the social security system.

It is also underpinned by a desire to keep a closer eye on foreigners after the Al Qaida-linked bombings that killed 191 people on March 11 last year.

Most of the Islamic militants accused of carrying out the attacks were born in Morocco and many of the victims were also immigrants.

To qualify, migrants must have an identity document, prove they were in Spain before last August, have a job contract for at least the next six months and have no criminal record.

Some 160 Social Security offices throughout Spain will open in the evening for the next three months to accept applications for residence and work permits, the government said on Sunday.

There have been long queues outside consulates in Spain and several Latin American countries in recent weeks by hopeful applicants or family members requesting documentation.

German and Dutch officials have raised concern over Spain's amnesty, fearing that, once legal, immigrants could move to northern Europe, the Financial Times reported on Friday.