The swoop on the San Salvador offices of Mossack Fonseca netted "a good amount of computer equipment," El Salvador's state prosecutor's office said on its Twitter account.
No arrests were made.
The authorities in El Salvador on Wednesday had announced a probe into whether the Salvadorans identified in the Panama Papers reports had broken any laws. Reports said some 33 Salvadorans were named.
The state prosecutor, Douglas Melendez, visited the law firm's premises on Friday.
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.
"We are going to carry out a complete investigation in compliance with the law," Melendez added.
He called on Salvadoran law firms that did business with Mossack Fonseca to come forward to speak with prosecutors.
The offshore companies of Mossack Fonseca's Salvadoran clients were used for transactions of hundreds of thousands of dollars, including to buy property in El Salvador "all under the radar of local authorities," El Faro, an online newspaper, reported.
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