The groups Save the Berkeley Post Office and Strike Debt Bay Area have planned a demonstration and teach-in on the privatization of public services starting at 1 p.m. in front of the post office at 2000 Allston Way.
The event will include speeches by activists and public officials, music and dance performances, street theater, free food, public forums and street art.
The Postal Service has said it plans to close the downtown Berkeley post office and hundreds of other post offices across the country because it is in poor financial shape due to the bad economy and a steady decline in mail volume.
The 52,000 square foot building, which was built in 1914, is on the National Register of Historic Places.
Berkeley officials including Mayor Tom Bates, Assemblywoman Loni Hancock, D-Berkeley, and Assemblywoman Nancy Skinner, D-Berkeley, have also said they will fight the closure.