Biti's Movement for Democratic Change (MDC), led by Prime Minister Morgan Tsvangirai, said it was the 12th time this year that militants, who include veterans from the guerrilla war of the 1970s, have tried to force their way to Biti's office.
"It is unfortunate that war veterans can invade government offices," Biti said in a statement.
He said he was powerless to change veteran allowances, as the War Veterans Act stipulates how much they are paid.
"For the act to be changed, it must first go through Cabinet and I cannot do it alone," he said.
Biti described the disturbance as a "low point" of the unity government formed over three years ago.
"These attacks are political," he said.
Biti castigated the police for failing to provide security for hundreds of civil servants who were locked inside their offices by veterans until late on Tuesday.
He said he had already met with veterans but said their demands could not be met by the cash-strapped government.
The militants also wanted to know how diamond sale proceeds were being used, but Biti said they should approach Mugabe's loyalists who are running the ministry in charge of mining.
Mugabe (88) is widely supported by war veterans in Zimbabwe and the former freedom fighters played a key role in the invasion of white-owned farms a decade ago. – AFP