The raids on properties across the capital, Kuala Lumpur, were related to investigations into state development fund 1MDB, they said.
Corruption claims against 1MDB were a major cause of Mr Najib's election loss to his former ally, Mahathir Mohammad.
Billions of dollars are unaccounted for from the fund, which Mr Najib set up.
Mr Najib himself was alleged to have pocketed $700m (£517m), which he has always denied.
He has been cleared by Malaysian authorities but is being investigated by several other countries.
Mr Mahathir has said he is considering reopening the case and that he believes the missing money can be recovered. He has banned Mr Najib from leaving the country.
The police raids on Mr Najib's office, private residence and several other properties linked to him in an upmarket area of the capital have been going on for several days, in the full view of the media.
At one point a locksmith was brought in to open a safe removed from Mr Najib's home.
On Friday, the head of the police commercial crime investigation unit, Amar Singh, told reporters the items removed included "284 boxes containing designer handbags".
"Our personnel checked these bags and discovered various currencies including Malaysian Ringgit, US dollars, watches and jewellery in 72 bags.
"Exactly how much jewellery, I would not be able to say, because we know that we confiscated bags containing jewellery and the amount of jewellery is rather big," Reuters quoted him as saying.
Mr Najib's lawyer has complained that the raids amount to "unwarranted harassment" and that the items seized "would seem of insignificant value".
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