Speaking in the capital Caracas, Mr Chavez said 100,000 Kalashnikov assault rifles already on order from Russia were not enough.
Venezuela needed a million well-armed men and women, he said.
Mr Chavez also likened US President George W Bush to the German Nazi leader, Adolf Hitler.
Diplomatic relations between Venezuela and the US have been strained, but they worsened earlier this week when both countries expelled one another's diplomats after Caracas accused the US embassy of spying.
The Venezuelan government has repeatedly accused Washington of trying to destabilise President Chavez - an allegation rejected by US officials.
'Defend our fatherland'
The BBC's Greg Morsbach in Caracas says the rally - to celebrate a failed coup led by Mr Chavez in 1992 - got off to a militaristic start, with a bugler heralding the arrival of President Chavez at the podium.
Wearing his trademark red army beret, Mr Chavez said Washington was considering invading Venezuela.
"I ask for permission ... to buy another cargo of arms because the gringos want us unarmed. We have to defend our fatherland," he said.
"Venezuela needs to have one million well-equipped and well-armed men and women."
Last year the US tried to block the sale of 12 Spanish military planes to Venezuela that were made with US technology.
But Madrid recently said it would go ahead with the sale using more expensive European parts.
Turning to oil, the president said if the Bush administration wished to cut diplomatic ties to Venezuela, he would have no second thoughts about closing all the Venezuelan refineries in the US.
"Let's see what'll happen to the price of crude oil then", Mr Chavez told his audience.
He said the US Defence Secretary Donald Rumsfeld had been wrong last week to compare the Venezuelan president with Adolf Hitler:
"The imperialist, genocidal, fascist attitude of the US president has no limits. I think Hitler would be like a suckling baby next to George W Bush."
Washington is deeply opposed to the government of left-wing Mr Chavez, who is a vocal critic of the US.
The US has expressed concerns about Venezuelan democracy under Mr Chavez and about the effect of his government's military purchases on regional stability.
But the US has not said it will break off relations with Venezuela, and correspondents say Washington has dismissed threats by Mr Chavez as inflammatory rhetoric aimed at his core supporters.