The raids began late on Wednesday night after Najib returned from evening prayers at a mosque and included two of his family residences in Kuala Lumpur. The other locations were other condominiums in his name and his prime ministerial office, the New Strait Times reported.
The raids continued till Thursday morning.
"No documents were taken, nothing of note, only personal possessions including bags. We believe that the police will take out two to three boxes of items," he said.
Following the searches, Najib accused police of harassment and disturbance. He also complained about the timing, saying the operation on the eve of the holy month of Ramadan denied his family's right to properly conduct their religious obligations.
However, Mahathir described the search as "police SOP (standard operation procedure)", saying they had good reasons to do so.
Najib, who is barred from leaving the country, is being investigated by the new government of Mahathir, his former mentor turned nemesis, over a multi-billion dollar graft scandal at the state-owned investment fund, 1Malaysia Development Berhad (1MDB), which he founded.
The fund is being investigated by the authorities of at least six countries, including the US Justice Department, which alleges that billions were laundered through layers of foreign bank accounts to finance Hollywood films and luxury goods including jewellery and a yacht.
Jeff Sessions, the US Attorney-General, described the scandal as "kleptocracy at its worst" and some of the missing money was alleged to have ended up in Najib's personal bank account.
The former Prime Minister has denied any wrongdoing and said the money was a donation from the Saudi royal family, which he had since returned.
However, accusations from the opposition that the scandal had blackened Malaysia's reputation abroad dogged Najib's election campaign and contributed to the downfall of his governing coalition, which had held power for six decades.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)