ATHENS—Greece's second-largest city, Thessaloniki, will become the battlefield for public anger against the government's reform efforts with mass protests expected to greet a keynote speech by Prime Minister George Papandreou on Saturday.

More than 5,000 police have been mobilized to barricade the city center ahead of Mr. Papandreou's speech, during which he's expected to talk about the urgency of the planned measures and stand firmly by government policy despite rising popular opposition, and even resistance within his ruling Socialist, or Pasok, party.

Greece's embattled government is scrambling to cut public spending after receiving stark ultimatums from other euro-zone governments that further rescue money will be withheld if Athens doesn't deliver on promises to reduce its budget deficit and overhaul its economy.

Without the aid, Greece is expected to run out of money within weeks, say senior Greek government officials.

The protests, which will involve at least half-a-dozen separate demonstrations, come amid a wave of walkouts planned by a number of workers—ranging from taxi owners to doctors and tax collectors—in coming days against these reforms.