A Guardia Civil spokesperson said the migrants were "massively entering from the sea" through the Tarajal breakwater but could not provide any estimates for the numbers.
State television TVE on Thursday reported that 2000 to 3000 youths had crossed while state news agency EFE said some had scaled the border fences without swimming.
Reuters was not immediately able to confirm those reports.
Ceuta, together with Melilla - another Spanish autonomous city located in North Africa - represents the European Union's only land border with Africa.
Both cities periodically experience surges in attempted crossings by migrants seeking to reach Europe.
Images showed some of the migrants shouting "Viva España!" - Long live Spain - as they entered the territory, many of them paddling across from the Moroccan side using inflatable inner tubes and other flotation devices.
Migrant rights activist Zakaria Zarroqui said Thursday's crossing was similar to one that occurred in May 2021 when about 10,000 Moroccan and sub-Saharan youths entered the enclave in days.
"The situation remains out of control as we speak," Zarroqui said, adding that people were still flocking to the Moroccan city of Fnideq in an attempt to cross into Ceuta.
Earlier this month, Spain's Supreme Court ruled that migrants intercepted at sea while attempting to reach Ceuta or Melilla cannot be summarily returned under the enclaves' special border-rejection regime.
"It has been a slow trickle since the Supreme Court's ruling but today it has been an explosion," a Guardia Civil spokesperson told Reuters.
Earlier on Thursday, Ceuta leader Juan Jesus Vivas urged the Spanish government to declare an emergency over the mass arrivals.
Ceuta's regional government requested that Spain deploy the army to guarantee safety and close the border with Morocco, in a message on X.
Spain's interior ministry said it was working closely with Morocco to address the surge in irregular arrivals, blaming people-smuggling networks for exploiting the Supreme Court's ruling to encourage undocumented migration.
Morocco's interior ministry did not immediately respond to a request for comment.
The mass crossing coincided with the arrival of more than 1500 migrants swimming into the city over the past week, according to regional authorities.
with AP
