At least 6,000 migrants have reached Spain's Ceuta enclave from neighbouring Morocco, a record number over a single day, Spanish officials say, APA reports citing BBC.
They say the migrants - who include about 1,500 minors - either swam around the border fences that jut out into the sea or walked across at low tide.
They are said to be mostly from Morocco. Spain says it has already sent some 2,700 back - but not the minors.
Spain's Ceuta and Melilla enclaves have become magnets for African migrants.
Spanish troops have been deployed to the beach to help border police at Ceuta's main entry point - Tarajal, on the enclave's south side.
Interior Minister Fernando Grande-Marlaska said 200 troops, plus 200 extra police were going to assist Ceuta's normal 1,100-strong border force. The enclave has some 80,000 inhabitants.
Later, Moroccan security forces at Fnideq, the adjacent town, fired tear gas to disperse a large crowd of migrants at the border fence, AFP news agency reported.
Prime Minister Pedro Sánchez has cancelled a trip to Paris - he was to attend a French-led summit on financial aid for Africa. Instead, he is focusing on the Ceuta crisis, and he promised "maximum firmness" in restoring normality to the enclave.