Saca, 51, and six other former officials from his government were detained in the early morning "on charges of various crimes," it said on Twitter.
Saca is a member of the conservative Nationalist Republican Alliance (ARENA). He was president of El Salvador from 2004 to 2009.
He is accused of embezzlement, money-laundering and links to illegal groups, the prosecutors' office said.
It give no further details of the accusations for the time being.
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He was seen as a strong ally of the United States during his time in power.
The other suspects arrested include former public waterworks official Cesar Funes, 46, and ex-presidential communications chief Julio Rank, 65.
El Salvador is a Central American country of six million people, bordered by Guatemala and Honduras.
It is stricken by poverty and violent crime involving drug gangs.
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