According to Spanish authorities, about 50,000 people had crossed into Ceuta by land and sea from Thursday in an unprecedented surge at one of the European Union's only land borders with Africa. More than 48,000 of them returned to Morocco within 48 hours, Spain said.
Reuters video on Saturday showed Spanish soldiers and police patrolling a mostly empty Tarajal beach shrouded in fog.
Civil Guard police started laying the new barrier at 7.50am local time on the Tarajal breakwater, one of the main points used by people trying to enter the tiny Spanish territory, the government said in a statement.
The pneumatic barrier, alongside a line of anchored naval buoys, is designed to stand 30 to 70 centimetres above water and extend as much as one metre below the surface, it added, while a channel between the barriers will allow Civil Guard vessels to patrol the area.
The mass crossing prompted alarm across the European Union, and 22 member states wrote a letter asking for coordinated action to protect external borders and other measures.
Ireland, which holds the rotating EU presidency, said it had called an emergency video conference meeting of the bloc's interior ministers to discuss the crisis on Tuesday.
Spain's centre-left Prime Minister Pedro Sanchez criticised Italy's decision on Friday to suspend passport-free Schengen travel arrangements with Madrid for one month.
"In the current international context, the EU cannot afford this kind of selfish, polarising and unlawful reaction," Sanchez wrote in a letter to the EU Commission and European Council presidents.
At least 67 bodies have been recovered on the Spanish side of the frontier, officials said, after some migrants drowned and others were crushed while trying to climb the breakwater and border fence.
Many were driven to migrate by economic hardship and encouraged by social media rumours.
Spanish Interior Minister Fernando Grande-Marlaska blamed people-smuggling networks for exploiting vulnerable migrants and spreading misleading interpretations of a recent Spanish Supreme Court ruling that migrants intercepted at sea cannot be summarily returned in the absence of a physical barrier.
Speaking from Ceuta, Grande-Marlaska told reporters Spain had largely restored order within 24 hours, but warned that the crisis was not yet formally over and that reinforced police and military deployments would remain as long as needed, though overnight crossings had fallen to about zero.
"Morocco isn't any threat to Ceuta or the rest of Spain - it's a fully reliable partner," he said, adding there had been no intelligence indicating a mass entry was imminent.
The AUGC union for Spain's Civil Guard said it was asking for a reinforcement of at least 200 officers, and warned that more work was needed to stop people swimming round the new inflatable barrier.
Madrid said the people who entered Ceuta irregularly could not travel onwards to mainland Spain or elsewhere in the Schengen zone.
In a statement, the foreign ministry added that travellers leaving Ceuta by sea or air undergo police identity checks, representing a second layer of control after the land border with Morocco.
In contrast to most of Europe, Spain has adopted a more open stance on migrants, introducing a program to grant residency to more than half a million undocumented people.
But it has rejected suggestions that the regularisation scheme had encouraged the rush into Ceuta, saying applicants must prove residence in Spain before January 1, 2026, and five continuous months of residence when applying.
