“A practice grenade containing a reduced explosive charge” was found in the morning within the grounds of the reception centre, a police spokesman told AFP, saying it was inside a plastic bag.   

“It looks like it might have been thrown in from outside,” he said, indicating the centre was immediately evacuated and anti-explosive experts brought in to detonate the grenade in a controlled explosion.

Located in the sprawling northeastern Hortaleza neighbourhood, the centre is home to scores of youngsters, most of them immigrants, with locals expressing concerns about it being overcrowded.

Last month, the centre hit the headlines during a televised election debate watched by millions when Vox leader Santiago Abascal claimed its presence had allegedly caused an increase in crime in the neighbourhood, where he himself lives.

Anti-immigrant rhetoric, particularly targeting minors, has been a key part of Vox's approach, with the party accused of falsifying and manipulating data to try to establish a supposed cause-and-effect relationship between illegal immigration and urban delinquency.

Shortly afterwards, Spain's human rights czar Francisco Fernandez Marugan issued a stern rebuke to those using “xenophobic and racist messages” to link migrant youths with crime, warning such rhetoric could be used to “justify acts of violence against them”.

“These children do not need to suffer any more violence; they have already been through enough in their lives,” tweeted Ione Belarra of the radical leftwing Podemos.

“Hatred spreads and you cannot control it. We will guarantee rights and fight against racism.”

Although November's election was won by the ruling Socialists, Vox scored huge gains, becoming Spain's third largest party. 

The Socialists and Podemos have agreed in principle to form a coalition government, but they need backing from other factions to pass an investiture vote — support which has yet to materialise.