An Interior Ministry spokesman said this was the largest number of immigrants ever to reach the islands aboard one boat.
There were some indications that the 25 meter (82 foot) boat might have set out from Senegal in West Africa, the spokesman said.
The occupants, all men in good health, were transferred to the island of Tenerife as El Hierro was too small to cater for so many immigrants, the spokesman said, the AP reports.
Thousands of people try to reach Europe through Spain each year, with an increasing number of them setting off from Mauritania and Moroccan-controlled Western Sahara. Many die during the dangerous crossing. Those who are intercepted are kept in holding centers. Authorities then have 40 days to repatriate or release them.
"There should be no Russian who goes to sleep without wondering if they're going to get their throat slit in the middle of the night,” Milley said