Madrid — There were long lines at registration points across Spain this week as application opened under a new amnesty program expected to see at least 500,000 undocumented migrants gain legal status to stay and work in the country. 

The program came into effect this month, offering undocumented migrants a one-year renewable residence permit if they can prove they have lived in Spain for at least five months and have no criminal record. 

The application window opened Monday and runs until the end of June.

But even as it gets underway, the program that sets Spain distinctly apart from other major European economies tackling the immigration issue with tighter restrictions, is itself facing major domestic challenges.

Why Spain's leader says the immigration amnesty is needed

Spain's government says the program will likely cover around 500,000 people, but estimates vary widely. 

The Spanish think tank Funcas says, according to the Reuters news agency, there are likely some 840,000 undocumented people working in Spain. The National Center for Immigration and Borders (CNIF), part of Spain's national police force, says between 750,000 and 1 million people in the country could ultimately apply. 

Undocumented migrants in Spain overwhelmingly come from Latin American countries such as Colombia and Venezuela, but there are also many from Africa and Asia. 

Socialist Prime Minister Pedro Sánchez has framed the policy as both a moral and economic necessity, arguing that many undocumented workers are already embedded in Spain's economy, and working in key sectors without legal protections.

The amnesty, he says, will safeguard those workers and help sustain Spain's economy and aging population.

Writing in The New York Times earlier this year, Sanchez contrasted his approach with more restrictive immigration policies elsewhere, writing: "Some leaders have chosen to hunt them down and deport them through operations that are both unlawful and cruel. My government has chosen a different way."

"Bypassing Congress is alarming" 

The program is set to face significant challenges from Sanchez' political opponents, who argue that Spanish workers should fill labor market shortages to ease the country's unemployment rate, which is one of the highest in the European Union.

The opposition also takes issue with the way Sanchez's government ushered in the policy — by royal decree. It meant parliament, where the prime minister's governing coalition lacks a majority, was bypassed.

Cayetana Álvarez de Toledo, a senior figure in the conservative People's Party, told CBS News the move raises serious concerns about the democratic process in Spain.

"Such an important decision needs a parliamentary debate. Bypassing Congress is alarming," she said.

On a practical level, Álvarez de Toledo said the measure could create a "pull effect," potentially putting more people at risk of victimization by organized criminal networks that traffic migrants and refugees. 

"If the message becomes: 'Make it to Spain and you stay in Spain,' that has obvious consequences," she told CBS News.

Álvarez de Toledo also warned that the government's "unilateral" move could affect Spain's relations with its fellow EU members, many of which have taken measures in recent years to restrict migration, especially undocumented migration.

Spain's opposition has pledged to fight the policy through multiple channels, and the far-right Vox party has already filed a formal request for the country's Supreme Court to suspend the royal decree while the measure is adjudicated.

Spain has implemented similar programs before, granting amnesty to undocumented migrants six times between 1986 and 2005, including under conservative governments. 

Whether this initiative eventually becomes a model for others to replicate, or a cautionary tale, may depend on how successfully the government can manage the volume of applications, and the political storm building around it.