Ivanishvili's supporters gathered in Tbilisi's Freedom Square for the rally, which was intended as a show of strength ahead of October's polls.
Former parliamentary speaker and government opponent Nino Burjanadze was among those participating in the rally.
Opposition leaders have accused Saakashvili of authoritarian tendencies, while others fear he may attempt to become prime minister in 2013 when his two permitted terms as president come to an end.
Police declined to estimate how many people took part in the rally. Some say it was the biggest rally since 2009.
Opinion polls suggest the alliance trails behind the governing party.