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 source said.
According to reports, judge José de la Mata ordered the seizure of 503 properties among them, 15 luxury residences in Puerto Banus as well as bank accounts holding more than €300 million
The assets of Rifaat al-Assad and his family in Spain are valued at €691 million ($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.
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.
The investigating magistrate has frozen 16 people's bank accounts and a further 76 accounts in connection with the case, the judicial source said.
Investigators say Rifaat al-Assad - first charged in France last June - holds some 90 million euros worth of property assets in France, including an office block in the eastern city of Lyon.
Rifaat al-Assad denies wrongdoing, saying much of his wealth stems from gifts bestowed by the Saudi royal family, which he says gave him political backing in the 1980s.
Last October, he told French authorities that former Saudi kings Fahd and Abdallah had provided those gifts, according to a source close to the French probe.
The initial probe was triggered by an activist group, which represents victims of financial crime.
Rifaat al-Assad insists he does not keep close tabs on his property holdings.