"There's going to be a huge march to the White House, and I guess that there will be lots of arrests," the spokesman said.
He also added that a mass show of civil disobedience and a 'die-in,' intended to symbolize the ongoing deaths in Iraq were planned.
Protesters intend to set up camp in the park opposite the White House Saturday evening. As camping is forbidden in the park, the majority of arrests are likely during this phase of the protest, the spokesman commented.
Anti-war demonstrators from all over the U.S. are due to arrive in Washington for the protest, which is set to begin at the White House at midday local time (4:00 p.m. GMT).
The protest comes as President Bush said Friday that he has accepted the recommendations of the top U.S. commander in Iraq, Gen. David Petraeus, regarding a gradual phase-out of troop escalation.
Five thousand troops will be withdrawn by Christmas, Bush promised in a speech that was broadcast live on primetime TV across the U.S.
The president also added that another 30,000 of the 168,000 U.S. troops on duty in Iraq are expected to return home by next summer.