In a televised message urging calm, Iraq's Prime Minister Nouri Maliki said he hoped that Saddam got "what he deserves" but said Iraqis should mark the verdict in a non-violent way.
Military leave was cancelled and a curfew was imposed in Baghdad and the flashpoint provinces of Diyala and Salaheddin, which includes Saddam's home town of Tikrit.
On the eve of the verdict, police said they had killed more than 50 insurgents in fierce fighting on the southern outskirts of the capital.
The interior ministry said four policemen also died and 16 insurgents were captured.
The verdict in the first of Saddam's trials is due to be handed down amid increased violence with 83 bodies, many showings signs of torture, found in Baghdad in the past days.
Deposed president Saddam Hussein and his co-defendants are accused of ordering the deaths of 148 Shias in the village of Dujail in 1982.
The trial comes more than three-and-a-half years after Saddam was overthrown by US-led forces.