The raids, in the Puerto Banus marina area of the plush resort of Marbella, are the result of a corruption investigation launched in France against Assad's uncle, Rifaat al-Assad.
The assets of Rifaat al-Assad and his family in Spain are valued at 691 million euros ($736 million), the source said, adding that "all these properties have been seized."
"Judge Jose de la Mata is coordinating the judicial operation against Rifaat al-Assad, uncle of the current president of Syria, for money laundering in Puerto Banus and Marbella", the source said.
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In 1984 Rifaat al-Assad was forced into exile for trying to overthrow his older brother, the late Syrian dictator Hafez al-Assad.
According to Spanish justice, Rifaat al-Assad, now aged 79, received 300 million dollars of state cash which he used to buy real estate in France, Liechtenstein, Luxembourg and Curacao.
French and Spanish authorities have been cooperating on the case and Spain opened an investigation on the back of the French one.
Madrid authorities say the Spanish probe established that Rifaat and his family had 503 properties in their portfolio, ranging from holiday villas to plush hotel apartments -- almost all in and around Marbella.
One residence, "La Maquina" at Benahavis just outside Marbella is worth an estimated 60 million euros.