The Panama-based law firm is at the centre of an international data leak scandal that has embarrassed several world leaders and shone a spotlight on the shadowy world of offshore companies, reports The Washington Post.
Documents and computer equipment were seized from the Mossack Fonseca office, officials said on Twitter.
Attorney General Douglas Melendez personally oversaw Friday's raid, his office said.
He told reporters that around 20 computers and a quantity of documents were confiscated, and seven employees questioned but not detained.
The authorities decided to conduct the raid when they observed staff removing the law firm's sign from outside the building, he said.
The Salvadoreans had used Mossack Fonseca to buy property in the country without declaring the purchases to the Salvadorean authorities.
The firm has denied the allegations and said the information is being presented out of context.
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