At least 57 people have died trying to reach the Spanish enclave of Ceuta after as many as 50,000 migrants entered the tiny territory from Morocco in what Spain’s prime minister, Pedro Sánchez, called “a violation of Spain’s territorial integrity”.

The huge influx, which took place over the course of just 24 hours, overwhelmed the city of 85,000 people, triggered a political crisis for Madrid and led to a mixed reaction from other EU countries, including calls for Spain to be suspended from the Schengen free-movement zone.

Sánchez, who arrived in Ceuta on Friday morning, said he would use all the resources at his disposal to guarantee the safety of the city’s population.

He attributed the mass crossing to “human trafficking mafias” that had taken advantage of a decision by Spain’s supreme court, which recently ruled that those intercepted at sea while trying to enter Ceuta, or Spain’s other north African enclave of Melilla, could not be summarily returned under special rules.

Sánchez said: “That has spread like wildfire during these last few hours through the networks of mafias that traffic in human beings and has produced an avalanche of the characteristics and dimension that we experienced yesterday.”

Sánchez said a temporary reception centre would be established to facilitate the documentation, registration, triage and “expedited” expulsion of people with no legal claim to be in Spain.

He also said the government would erect a visible barrier – a line of buoys – near the breakwater at Ceuta’s Tarajal beach to make it clear where the border was, and to legally comply with the court’s ruling.

Although the Spanish government said about 50,000 people had crossed the border on Thursday and in the early hours of Friday, the president of Ceuta put the figure at about 60,000.

Spain’s interior ministry, which has deployed additional police officers and soldiers to the enclave, said more than 48,300 people had voluntarily returned to Morocco by 6pm on Friday.

The latest incident dwarfs the last mass border crossing in May 2021, when Morocco loosened its border controls, allowing about 10,000 people to enter the territory over the course of two days.

That crossing came amid heightened tensions between Madrid and Rabat over Spain’s decision to allow the leader of the Western Sahara independence movement, the Polisario Front, to be treated for Covid-19 in Spain. Morocco and the Polisario Front both lay claim to Western Sahara, which was under Spanish rule until 1975, when colonial authorities handed over the territory to Morocco and Mauritania.

That diplomatic crisis ended in 2022 when Madrid reversed its longstanding policy of neutrality and backed Morocco’s plan for Western Sahara – a move that had caused a diplomatic rift between Spain and Algeria, which had supported the Polisario group.

Sánchez visited Algiers on 20 July – the first such visit by a Spanish head of government in four years – marking a thaw in relations with Algeria.

Moroccan government sources told the Europa Press news agency that the surge of people entering Ceuta was down to “criminal organisations” and insisted that Rabat’s cooperation with Spain remained “exemplary”.

Rachid Sbihi, who leads a local workers’ association representing officers of the Spanish Guardia Civil – which patrols Spain’s borders – described the situation as a “serious humanitarian crisis”.

Without shelter, thousands of people, including unaccompanied children, had been left sleeping in parks and on pavements, while others roamed the streets aimlessly. “It’s chaotic,” said Sbihi.

Among those who died, many drowned, but some were killed in a crush to cross the breakwater fence at Tarajal beach, Sbihi said.

Video footage on Thursday showed crowds of people walking around the breakwaters at Tarajal beach on to local roads. While most appeared to be young men, there were also families with women and small children.

Ceuta and Melilla have the E​U’s only land border with ​Africa. ‌Both cities ​periodically ​experience surges in attempted crossings by people seeking to reach Europe.

The events in Ceuta led some EU countries to issue strong threats or rebukes.

Spain’s foreign minister, José Manuel Albares, summoned Italy’s ambassador to Madrid on Friday after Giorgia Meloni’s far-right government called for Spain to be suspended from the Schengen free-movement zone.

Italy’s foreign minister, Antonio Tajani, blamed the situation in Ceuta on the Sánchez government’s decision to offer a now closed scheme to regularise the status of more than 1 million undocumented migrants and asylum seekers. He said the programme had incentivised human trafficking and was “profoundly wrong”.

Tajani’s remarks prompted a sharp response from Albares, who said the comments were “unbecoming” and accused Italy of “partisan demagoguery”.

Italy announced on Friday evening that it would be suspending its Schengen arrangements with Spain, but the practical impact of the move was not immediately clear.

Meloni said the measure would be kept in force only as long as needed, adding: “In parallel, Italy is ready to support every European initiative to assist Spanish institutions as needed to restore full control of the union’s external borders and to address the ongoing situation with determination.”

Responding to Italy’s unilateral decision, a European Commission spokesperson said a member state “may exceptionally and temporarily reintroduce internal border control as a matter of last resort, to address serious threats to public order or national security”. But they added that the member state would need to notify the commission and explain how the situation in Ceuta constituted such a threat.

The Guardian understands that Italy has not yet formally notified any of its EU partners of its decision.

The Swedish prime minister, Ulf Kristersson, urged Spain to “act swiftly to regain control of the situation and fulfil its commitments within Schengen cooperation”.

The Finnish interior minister, Mari Rantanen, said that as Spain had “completely failed to protect the Schengen area’s external border”, all EU countries should support Italy’s proposal to suspend its Schengen status.

However, in a statement later on Friday, the European Commission said Ceuta and Melilla were subject to special Schengen rules that prevented onward movement within the zone.

The EU’s migration commissioner, Magnus Brunner, called the situation in Ceuta “unacceptable” and said he was in close contact with the Spanish authorities. But he also confirmed there was no onward route from Ceuta given the enclave’s unique status under the Schengen rules.

He said: “The Schengen borders code provides for special rules that apply to Ceuta and Melilla. Border checks on persons departing from Ceuta and Melilla remain in force.”

But there were also shows of unity. The European Council president, António Costa, expressed his “solidarity with Spain in the current situation of illegal migration in Ceuta”.

The French president, Emmanuel Macron, said he had been in touch with the Spanish government to express his solidarity and to offer assistance to Spain. But, as a precautionary measure, Macron said he had asked the interior ministry to strengthen the French controls on its border with Spain with four mobile units and drones.

In Spain, the opposition People’s party (PP) called on Sánchez to come up with a tough response, saying the military and police reinforcements announced on Thursday night were insufficient.

In a post on X, the PP’s leader, Alberto Núñez Feijóo, wrote: “People have continued to arrive in Ceuta throughout the night, and those on the ground describe an occupied city, with nothing sufficient being done to prevent it.”

The far-right Vox party called the incident “an invasion”.