The cabinet on June 2 approved the special budget of 610 billion Taiwan dollars (18.2 billion US) for the purchase of advanced weaponry amid tensions with China, but the plan has sparked an outcry.
Some critics say Taiwan cannot afford the expense while others say the new weaponry will not be delivered in time to help Taiwan fend off any Chinese attacks in coming years, which the defense ministry has warned of.
"If we agree it is wrong to leave such a huge financial burden to our children, then we should do something," Ho Tsung-hsun of the No Nuke Union Taiwan said Wednesday.
The government's debts have accumulated to some 7.6 trillion Taiwan dollars (226.87 billion US), opposition parties said.
Ho, one of the organisers, said they expected hundreds of people to turn out for Saturday's march.
"Response to our march plan is stronger than expected" after it was unveiled on the Internet Monday, he told AFP.
Ho said they had to make the government aware of opposition to the special budget, which calls for the procurement of eight submarines, a modified version of Patriot anti-missile systems PAC-III and a fleet of anti-submarine aircraft over a 15-year period beginning in 2005.
Parliamentary speaker Wang Jin-pyng confirmed he is to lead a delegation to the United States Thursday for the mega arms sales.
US President George W. Bush offered the sales in April 2001 as part of the most comprehensive arms package to the island since 1992.
Taiwan Vice Defense Minister Tsai Ming-hsien said early June that mainland Chinese forces might attack the island some time between 2006 and 2008.
China has repeatedly threatened to invade Taiwan should the island declare formal independence, although the two sides split in 1949 at the end of a civil war.