Videos and images showed thousands of people swimming into the city on Thursday, while local media reported crossings continuing overnight.
Ceuta, on Morocco’s northern coast, is separated from mainland Spain by the Strait of Gibraltar and has long been a focal point for migrants attempting to reach Europe.
Local officials had appealed to Madrid for help after a recent rise in attempted crossings, but there were chaotic scenes on Thursday as border controls apparently broke down.
The astonishing estimation by Spanish officials suggests the number of migrants who entered into Ceuta in the last day or so could be more than half of the population of the city – which sits at around 83,600 according to the latest local figures.
Spain later deployed its armed forces to bolster security in Ceuta and at its sister city of Melilla, where between 300 to 400 crossings were also reported overnight.
Speaking on Friday, the prime minister condemned the crossings and said it was an attack on Spain’s territorial integrity.
Sánchez added that the government would be looking at reinforcing the border with Morocco.
He said authorities in Morocco were cooperating with plans to return those who entered the territories illegally.
Coveted by Morocco, both cities have long been a flashpoint in diplomatic relations with Spain. Madrid asserts that both territories are integral parts of Spain and have the same status as the semi-autonomous regions on its mainland.
Both territories trace their Spanish past to the 15th Century, and they have enjoyed a limited degree of self-government as Autonomous Communities since 1995.
They form the European Union’s only land borders with Africa.
One migrant told Reuters news agency that he was returning to Morocco, adding:
“It’s not good at all, and it’s not fun either.
“I mean, people are dying here. I beg you, please, if you haven’t come yet, don’t come. Don’t do it.”
Local media reported chaos on the streets of Ceuta since Thursday, with one resident telling Spain’s ABC that people are scared.
“Nobody is going out on the street. They’re afraid.
“This is stronger and more aggressive than what we’ve experienced before,” a hospital security guard told the Spanish site.
Another person who spoke to the outlet said the people crossing into the territory were not “poor or needy”.
“They are coming to Spain to abuse the system,” he said.
Yassine, a 24-year-old migrant, told the outlet he could not go back to Morocco.
“I have nothing there, Morocco is death,” he added.
The sudden influx of people on Thursday followed several days with an increase in arrivals swimming into Ceuta. It appears that word of their success spread quickly.
The last time there was a surge in arrivals – though on nowhere near this scale – was in 2021. At that point, the Moroccan authorities allowed some 8,000 people to cross the border following a dispute with Madrid over the sovereignty of the Western Sahara.
That diplomatic dispute has flared again recently, after Prime Minister Pedro Sánchez travelled to Algeria which backs independence for the region.
Spain’s interior ministry said it was working with Morocco to ensure the return of all who crossed illegally into its territory “as soon as possible”.
Sánchez is set to visit Ceuta later on Friday.