Thousands of migrants, many of them minors, have swum or sailed around the EU's land border with Africa. Some believe Morocco deliberately hobbled its border control measures.
Around 5,000 people crossed the border from Morocco into the Spanish exclave of Ceuta on Monday, a single-day record, Spanish officials said.
The surge came as Morocco lodged a diplomatic protest against Spain for accepting a rebel leader for medical treatment.
Thousands of young migrants swam or waded around the heavily fortified border land border. Others floated across in inflatable dinghies or swimming rings.
By Tuesday morning, around 6,000 people had crossed the border into Ceuta, the Spanish government said, including 1,500 people thought to be teenagers.
At least one person drowned while attempting to cross, according to local media reports.
The arrivals were checked by the Red Cross and taken to a migrant reception center.
The Spanish exclaves of Ceuta and Melilla are Africa's only land border with the European Union, making it a target for people seeking to migrate to Europe. Only 475 migrants had crossed into Ceuta between January 1 and May 15 this year.
Spain deployed its military to the Moroccan border on Tuesday after the group crossed over. Several military armored vehicles parked on Tuesday at Tarajal beach in Ceuta, where the border fence leads to a short breakwater extending into the sea.
"It's such a strong invasion that we are not able to calculate the number of people that have entered,'' said the president of Ceuta, Juan Jesus Vivas. "The army is in the border in a deterrent role, but there are great quantities of people on the Moroccan side waiting to enter."
Later on Tuesday, Spain announced that it had returned half of the migrants who entered Ceuta. Speaking after the weekly cabinet meeting, Interior Minister Fernando Grande-Marlaska said Spain had already returned "2,700 people" who had entered the territory illegally, updating an earlier figure of 1,500.
The EU Commissioner for Home Affairs, Ylva Johansson, on Tuesday urged Morocco to "prevent irregular departures" of migrants. Johansson, speaking to the European Parliament, called the migrant arrivals to Ceuta "unprecedented" and "worrying," and noted "a very substantial number of them [are] children."
"The most important thing now is that Morocco continues to commit to prevent irregular departures," and that those that do not have the right to stay are orderly and effectively returned, she said, adding: "Spanish borders are European borders."
Local officials anonymously accused Moroccan border patrols of deliberately lax controls amid a diplomatic spat.
Rabat has clashed with Spain in recent weeks, after it emerged that Polisario Front leader Brahim Ghali has been receiving treatment for COVID-19 in northern Spain since mid-April.
The Polisario Front militants have fought to secede Western Sahara, annexed by Morocco in 1976. Analysts have warned the spat could threaten bilateral cooperation on fighting illegal immigration. Spain has justified treating him on humanitarian grounds.
Last month, the Moroccan foreign ministry said the stance was "inconsistent with the spirit of partnership and good neighborliness" and later warned the move would have "consequences."
Mohammed Ben Aisa, head of the non-profit Northern Observatory for Human Rights, told the AP that the recent diplomatic tensions played a role in the influx, in addition to good weather and the seasonal attempts to reach Europe.
"The information that we have is that the Moroccan authorities reduced the usually heavy militarization of the coasts, which come after Morocco's foreign ministry statement about Spain's hosting of Brahim Ghali," he said.
"The area is heavily monitored by security forces and attempts there, whether to climb the fence or swim, are usually stopped," he added.
Footage published in Spanish media showed people climbing the rocky breakwaters and running across the Tarajal beach, in the southeastern end of the city. Other videos showed long lines of young men outside Red Cross warehouse, waiting to be registered with authorities.
On Monday, the Spanish interior ministry said in a statement that Spain "has been working tirelessly on a migration policy that concerns the whole of the European Union and Morocco, the country of origin of the people who have arrived swimming today."
It has deployed hundreds of officers in light of the arrivals to help process and return them more quickly.
Rabat cooperates with Madrid on migration flows in exchange for recurring payments and training for its police and army. The EU also relies on Moroccan intelligence for fighting extremism.
Spain does not grant Moroccans asylum status, but it does allow unaccompanied migrant children to legally remain in the country under the government's supervision.
lc, aw/msh (AP, AFP, Reuters, EFE)