The stoppage is set to continue through Monday morning.
The workers say that personnel management at the plant is inflexible and one-sided in its interpretation of the current labour contract. The employees say that the plant does not have enough workers for the amount of work that is expected to be done.
UPM closed the pulp mill at Tervasaari in late 2008. The employees say that about 250 people have lost their jobs as a result.
The paper mill at Tervasaari employs 320 paper workers, and about 100 white collar employees.