Thousands of migrants have descended on consulates across Spain after the country approved plans to give legal status to 500,000 migrants.

Since Saturday, some 8,000 migrants have visited the Moroccan consulate in Almeria alone to collect the necessary documentation needed to gain legal entry into the country.

It comes as online applications opened today after Spain's socialist government rubber-stamped the initiative at a cabinet meeting on Tuesday.

Images from this weekend from Madrid, Bilbao, and Almeria showed snaking queues consisting of hundreds of people in the streets.

In a bid to alleviate pressure on an already overburdened immigration system, only five of the country's 54 immigration offices are responsible for handling applications. 

The rest are being distributed among social security offices, post offices, and NGOs, according to the Spanish union CCOO. 

Immigration offices across the country this week threatened to strike next week in protest at Prime Minister Pedro Sanchez's mass amnesty program for undocumented migrants.

The amnesty is a central plank of Sanchez's progressive agenda to harness the economic benefits of migration for its ageing population, even as other European governments move to tighten their borders.

Immigration officers warned that the system is unprepared for the challenge and have threatened a strike from April 21, halting all immigration applications in protest at the lack of resources allocated to the process.

'The government is once again implementing a new regularization without giving offices enough economic resources to handle it,' Cesar Perez, a union leader for Spain's immigration officers, said to Reuters earlier this week.

Spain's 50 million-strong population has swelled in recent years to include around 10 million people living in Spain who were born abroad.

Spanish think tank Funcas estimates that roughly 840,000 undocumented migrants are in the workforce at present.

The country's opposition Popular Party has deemed the drive reckless, despite former conservative governments pushing through similar measures.

Isabel Diaz Ayuso, president of the community of Madrid and a prominent figure in the party, has threatened to appeal the drive in court.

Sanchez described the drive in a letter addressed to citizens published on Tuesday on X as not only an act of justice but also an economic necessity.

'Spain is ageing... Without more people working and contributing to the economy, our prosperity slows, and our public services suffer,' he wrote.

In the past three years, Spain's population rose by 1.5 million to 48.9 million, with almost all the increase due to immigration.

Latin Americans make up 70 per cent of recent arrivals.

Sanchez argues immigrants are key to Spain's economy, which expanded 2.8 per cent last year - more than twice the average expected in the entire eurozone.

The country has been outperforming other EU nations in recent years, with unemployment - a longstanding issue in the Spanish economy - dipping below 10 per cent for the first time since 2008.

But with about 90 per cent of new jobs going to immigrants, income per person has barely grown in Spain.

Moreover, each year sees 140,000 new households, but only about 80,000 new homes built.

A lack of affordable housing has become a central grievance among voters, contributing to social tension.

Critics of the new programme argue that without simultaneous housing policy reforms, legalising large numbers of migrants increases competition for scarce accommodation, particularly in urban centres such as Barcelona and Madrid.

Santiago Abascal, the leader of the populist hard-right party Vox, accused the Socialist-led coalition of accelerating what he called an 'invasion'.

Pepa Millán, spokeswoman for Vox, said the plan 'attacks our identity', pledging that the party would appeal before the Supreme Court in an attempt to block it.

The political row escalated after Musk posted a link on X - which he owns - to a post by a man named Ian Miles Cheong who called the plan 'electoral engineering', along with the comment: 'Wow.'

'Spain just legalized 500,000 illegal aliens to "defeat the far-right",' Cheong wrote in the post which has had over 16 million views.

'The logic is simple: legalise half a million people, fast-track them to citizenship (which takes as little as two years for many), and you've effectively imported a massive, loyal voting bloc that's indebted to the left,' he continued.

Sanchez hit back at Musk, responding to the tech mogul's post on X with the message: 'Mars can wait. Humanity can't.'