A process is provided to partially upgrade heavy oil using two or more reaction zones connected in series, each reaction zone being a continuous stirred tank maintained at hydrocracking conditions. The heavy oil feedstock and a solid particulate catalyst are stirred to form pumpable slurry which is heated to a target hydrocracking temperature and then continuously fed to the first reaction zone. Hydrogen is continuously introduced to the reaction zone to achieve hydrocracking and to produce a volatile vapor stream carried upwardly by the hydrogen to produce an overhead vapor stream. The hydrocracked heavy oil slurry from one reaction zone is fed to a next reaction zone also maintained under hydrocracking conditions with a continuous hydrogen feed to produce a volatile vapor stream. The overhead vapor stream from each reactor zone is continuously removed, and the hydrocracked heavy oil slurry from the last of the reaction zones is removed.