Almost 900 African migrants reached the Canary Islands by boat on Tuesday, a record number for a single day.
Many undertake dangerous sea journeys in rickety boats
Nine boats with 898 people made it to the archipelago, rescue workers said.
The surge has prompted the head of the Canaries' government to call the influx Spain's worst humanitarian crisis since the civil war of the 1930s.
Already, more than 20,000 migrants, mostly from sub-Saharan Africa, have arrived on the islands this year - double the previous annual record.
Estimates of the number that have died en route to the islands located off the west coast of Africa range from 590 to 3,000.
The Canaries have become a main point of entry for illegal immigrants seeking to reach the European Union, following a crackdown on migration to the north African Spanish enclaves of Ceuta and Melilla in 2005.
Tuesday's record influx came after a weekend in which more than 1,400 Africans landed on the islands.
The Canaries have seen four times as many migrants this year as last
"The dam is about to burst," the head of the regional government told members.
"How many more victims, how many pictures of exhausted bodies?" Adan Martin asked.
Opinion polls suggest immigration has become a key concern for Spaniards.
A survey for Instituto Opina released on Wednesday indicated a vast majority thought too many illegal immigrants were arriving, the government was failing to cope and the EU should get more involved.
The government has said it will not tolerate the continued arrivals.
On Monday, Deputy Prime Minister Maria Teresa Fernandez de la Vega called on African countries to help cut off the flow of migrants, and to take them back again.
Spain has accords with several African countries under which they have agreed to tackle illegal migration and accept the repatriation of their nationals, in exchange for development aid.
Spain has been criticised by European Commission Vice President Franco Frattini for a 2005 amnesty for 600,000 illegal immigrants, on the grounds that it encourages more people to try to reach Spain.
The migrants take to the seas crammed into open wooden boats for a crossing of up to 10 days. An unknown number do not make it.
Africans make up only a small proportion of the more than half a million people migrating to Spain every year, but they put strains on the resources of the Canary Islands.
On Monday, officials said internment centres which had room for 5,446 people were already holding 5,461 - while another 700 were being held in police stations.
Thousands of migrants have already been transferred to the mainland this year, and some have been sent home.
Senegal suspended repatriation of its citizens earlier this year, after migrants returned claiming they had been maltreated.
Last week Spain called on the European Union for more help to deal with the crisis.
Experts from several EU countries have been helping identify the nationalities of migrants landing in the Canary Islands since July.
Ships from Portugal and Italy and a Finnish aircraft are also supposed to be helping patrol the West African coast.
But Spain says more boats, planes and personnel are needed.