
The early voting process involves 625 police personnel and 238 military personnel, as well as their spouses, who will cast their votes to elect their new representative at the state assembly.
The early voting centres will close at 5pm.
The by-election sees a four-cornered fight between Pang Sock Tao (Pakatan Harapan), Khairul Azhari Saut (Perikatan Nasional), Hafizah Zainuddin (Parti Rakyat Malaysia) and independent Nyau Ke Xin.
It is being held following the death of the incumbent Lee Kee Hiong, 58, due to cancer on March 21.
The Kuala Kubu Baharu state constituency has 40,226 registered voters.
The Election Commission has set polling for the state by-election on Saturday.