Spanish intelligence services warned of social media calls for a mass migrant crossing into Ceuta the day before tens of thousands of people entered the territory, newly released government documents show.

The revelations emerged in documents declassified by the Spanish government amid accusations it failed to give the full story of the migrant influx.

On 29 July, Spain’s National Intelligence Service (CNI) told Moroccan and Ceuta authorities that social media groups were encouraging migrants to cross the following day.

Prime Minister Pedro Sánchez has insisted that the events of 30 July were impossible to predict, but opposition parties say the documents show the government misled Spaniards.

Sánchez had previously told parliament that there was “no information that conveyed the magnitude” of the “crisis we were going to suffer” on the last two days of July.

At least 72,000 migrants crossed from Morocco into Ceuta on 30 and 31 July. Most swam around a border fence and at least 100 people died during the crossing, according to figures from Ceuta’s mayor.

Although most returned, as many as 5,000 remain in makeshift camps or are sleeping on the city’s streets and beaches, leading to unrest in the autonomous Spanish city in North Africa. Locals have expressed safety concerns and some have destroyed shelters and blocked aid being supplied to the migrants.

In a note on 29 July, the CNI told Moroccan intelligence it had identified WhatsApp and Facebook groups promoting an attempt by migrants to reach the Spanish exclave the following day, “both swimming and via the [border] fence”.

The CNI reported that around 180,000 people were receiving the messages, one of which read: “Whoever wants to cross has the last chance on Throne Day [30 July] at 8 the authorities will be distracted with the celebrations and parades.”

It also warned, via text message, the central government’s representation in Ceuta and the civil guard about the social media activity.

The messages are part of a tranche of more than 40 security documents linked to the Ceuta crossing that the government declassified following mounting pressure on Sánchez over his handling of the crisis and widespread allegations in Spain that Morocco had played a role in facilitating the crossings.

Earlier this month, a leaked Spanish police report seemed to implicate Moroccan police in the incident.

However, Sánchez has maintained that there is no evidence suggesting Moroccan complicity.

Shortly before the documents were released on Wednesday, he told Spanish television that “the mass arrival of migrants was not expected”.

But opposition politicians said the newly released material contradict that claim.

“They had the information, they are directly responsible for the invasion of Ceuta and for those who died,” said Ester Muñoz of the conservative People’s Party.

Jorge Buxadé, of the far-right Vox, said on social media that the documents showed that Sánchez “and his band of criminals should be brought before the Supreme Court for treason and security crimes”.

The government has continued to insist that it acted according to the information available.

Foreign Minister José Manuel Albares said he was “not aware of any report that might have forseen the magnitude of what happened on July 30”.

A government source said the published documents “showed that the information available before the mass crossing did not make it possible to anticipate the scale that the crisis would reach or prepare with enough time the necessary response”.

Miguel Ángel Pérez Triano, the central government’s delegate in Ceuta, addressed the revelations about the text messages the CNI had sent to his office the day before the mass crossing.

“The Whatsapps from [July] 29 do not constitute an alert or a report,” he said. “It is nothing more than an exchange of information.”