Portugal's interior minister has resigned following an investigation into alleged corruption linked to the allocation of residence permits.

Miguel Macedo said he was not implicated in the scandal but was resigning to preserve the credibility of state institutions.

Police arrested 11 people, including the head of the country's immigration service, last week.

Portugal provides "golden visas" to foreigners willing to invest. The main beneficiaries of the fast-track permits have been Chinese citizens who have been investing heavily in Portuguese property.

Police said the 11 people detained were suspected of corruption, money-laundering, influence-peddling and embezzlement.

Macedo told state television he had "had no personal responsibility" in the case.

"This is a decision exclusively based on personal and political convictions," he said. "I leave to defend the government and the state authority and the credibility of the institutions."

Searches were carried out last week at several locations, including the interior ministry.

Manuel Palos, head of the country's immigration service, has been questioned by a judge on suspicion of accepting bribes, his lawyer said.

Meanwhile, Portugal's government has defended the visa scheme, which in two years has brought in over 1bn euros ($1.3bn) in investment.