Bush hopes to send "a signal of unity" to the world
Mr Bush has also been given final congressional approval by the House of Representatives to "use all necessary and appropriate force" against those behind the hijack attacks on Washington DC and New York.
Earlier, the US Senate gave approval for a $40bn package for anti-terrorism measures and to help rescue and recovery work in New York and Washington.
In a statement the US Defense Department said: "Secretary of Defense Donald Rumsfeld has requested and President Bush has approved an order to call up to 50,000 reservists to active duty."
The Pentagon said 13,000 air force reservists would be called up initially along with 10,000 army, about 7,500 marines, 3,000 navy and 2,000 coast guard troops.
Defence officials said that the initial forces to be called up - including fighter jet pilots and crews to protect US cities - would be for "homeland defence."
Retaliation
But they stressed additional moves could be made to mount a war against both those who crashed hijacked airliners into US buildings on Tuesday and countries that support them.
The $40bn package - agreed earlier by White House officials and leaders of Congress - is double the amount originally requested by President Bush.
The Senate cut through debate and usual parliamentary procedures to rush the money through.
Half of the fund is expected to go to those directly affected - primarily in New York and the Washington area.
Money has also been earmarked for the costs of the rescue mission, reconstruction and improved security.
Display of unity
But the broad language of the bill will also allow President Bush to use the funds to retaliate against those responsible for the attacks.
The Senate majority leader, Tom Daschle, said there was a unanimous understanding that this week's package was a minimal down payment on what would be required.
"This body will provide whatever resources are needed to respond to this challenge, not just today, not just tomorrow, but for as long as it takes," said Republican Congressman David Obey.
Correspondents say the speed with which details were agreed - a process which usually takes weeks - was clearly intended to show that political differences had been swept aside in the face of the crisis.
Mr Bush had reminded officials of this as he made his request, saying that to pass this bill without delay would "send a powerful signal of unity to our fellow Americans and to the world".
Key suspect
The funds can be used to "counter, investigate or prosecute domestic or international terrorism" and for "supporting national security".
On Thursday, US Secretary of State Colin Powell confirmed for the first time that Islamic militant Osama Bin Laden was a key suspect.
He is widely believed to be sheltering in Afghanistan, controlled primarily by the Taleban.
On Friday, federal authorities made their first arrest in the worldwide investigation into the attacks, said an unnamed government official.
The suspect has not been identified, but is described as a material witness.