Pressure is mounting on Poland's government over a visa fraud scandal that has led to seven people being detained and the dismissal of the deputy foreign minister.

The opposition has claimed that thousands of work visas for migrants were fast-tracked without proper checks in exchange for large sums of money. The scheme was reportedly run through Polish consulates and intermediary companies.

The allegations could prove damaging for the ruling nationalist Law and Justice (PiS) party, which is campaigning on an anti-immigration platform ahead of parliamentary elections next month.

How big was the alleged visa fraud scheme?

Poland's government has accused the opposition of exaggerating the scale of the issue and says it is waiting for the results of an investigation.

Authorities have said the scheme may have involved several hundred Polish work visas being issued in countries in the Middle East, as well as India, the Philippines, Singapore, Hong Kong and Taiwan.

But the Opposition said the actual number could be much higher. Donald Tusk, leader of the opposition Civic Platform party, said some 250,000 Polish work visas had been issued in Africa and Asia over the past 30 months in exchange for bribes, although he did not specify where his information came from.

He called it "probably the biggest scandal in Poland in the 21st century."

Donald Tusk says as many as 250,000 visas were issued in exchange for bribes, but the government argues that's an exaggeration Deutsche Welle

Polish media, citing sources in the Foreign Ministry and PiS, reported that the scandal came to light after other EU states noticed an unusually high number of migrants entering with Polish visas.

Migrants with these visas are meant to stay and work in Poland. However, they can also travel to other countries within the EU, such as Germany, because Poland is part of the border-free Schengen area.

What has the fallout been?

At the end of August, when the scandal first surfaced in the media, the deputy foreign minister in charge of consular matters, Piotr Wawrzyk, was sacked. His department was also raided by Poland's anti-corruption agency.

On Thursday, prosecutors said seven people, none of them state officials, had been detained over the alleged visa fraud.

On Friday, Reuters news agency reported that the Foreign Ministry's legal department head was fired.

Government under pressure ahead of vote

The scandal comes at a difficult time for the ruling PiS party, which is seeking reelection for an unprecedented third term in the October 15 vote.

Although it is leading in opinion polls, it will likely fall short of winning enough support needed for a majority.

The party's anti-immigration stance has been a key part of its campaign. The government has repeatedly refused to admit migrants under an EU burden-sharing program. It has also built a steel wall along the border with Belarus to block migrants from entering.