(CNN) -- Spain sent police reinforcements to its enclave of Melilla, on Morocco's north coast, on Wednesday, after more than a thousand would-be immigrants to Europe from Mali and Cameroon rushed en masse to try to illegally enter Spain.
Under cover of dense fog, on Tuesday just before 8 a.m., the young men climbed a section of the six-meter-tall, chain-link fence that had not been reinforced, while Moroccan and Spanish security forces tried desperately to prevent them.
About 500 made it in, and they were soon running toward an overcrowded immigrant holding center in Melilla, shouting "bosa, bosa," which means "victory, victory."
It was the biggest "assault" in nearly decade, surpassing the 350 immigrants who scampered inside Melilla on a single day back in October 2005, authorities said.
On Wednesday, Madrid sent 120 additional police and civil guards to Melilla.
The Spanish government estimates there are 40,000 African migrants gathered in northern Morocco who want to enter Melilla or the other Spanish enclave, Ceuta, hundreds of miles to the east and close to the Strait of Gibraltar.
The government estimates an additional 40,000 migrants are gathered close to the border of Mauritania and Morocco, with the intention of moving north to try to enter Spain.
On March 6, Spain's Interior Minister Jorge Fernandez Diaz, on a visit to Ceuta, said the mass influx of immigrants was not just Spain's problem, but Europe's, and he called for more EU cooperation.
The immigrant-holding center in Melilla now has about 1,800 residents -- more than triple its capacity.
After Tuesday's latest influx, the Spanish Red Cross and the army hastily erected 26 tents and expanded the perimeter of the center, where the migrants get meals, showers and medical care, said Mariano Fernandez of the Red Cross.
Spanish media reported on Wednesday that the government would soon need to transfer dozens or hundreds of the immigrants to the Spanish mainland. That would put them closer to other EU countries, like France, which have minimized controls at many internal EU borders such as the Spanish-French crossings. But the government had no immediate comment about potential transfers of migrants.
The latest influx in Melilla comes as controversy swirls around the events of last February 6, when some 15 migrants drowned while trying to swim from Morocco past a retaining wall and into Ceuta.
Spanish police fired rubber bullets on that occasion, and rights groups allege this may have contributed to the deaths. The government says the rubber bullets were not aimed directly at the migrants. Later on Wednesday, a top Interior Ministry official is due to offer further explanations before Spain's parliament.