Security has tightened across the US after 11 September
The authorities had been alerted by a mail carrier who became alarmed by an attached note - which read "Free Palestine" and also mentioned al-Qaeda and Osama Bin Laden.
No one was harmed in the blast, although shrapnel was sprayed up to a 100 feet (30 metres) away.
News of the package bomb came as the White House announced tighter security measures in the wake of a threatened terrorist attack on US nuclear installations on American Independence Day.
Explosive device
Federal agents in America are looking into the origins of the explosive device in Philadelphia, which went off on Monday as a bomb disposal squad was attempting to defuse it from a distance.
The package is believed to have had a sophisticated switch mechanism to allow the bomber to avoid harming himself.
A second suspect package was found on Tuesday in the same part of Philadelphia and caused initial concerns, but may have been a false alarm.
Postal workers have been on alert since six pipe bombs exploded in letter boxes in Iowa and Illinois earlier this month, injuring six people.
A student has been charged in connection with those explosions.
Nuclear plant threat
White House officials say they have also received news of a further threat to national security.
The government says it has learned that terrorists are threatening to attack an American nuclear power plant on 4 July, US Independence Day.
Since 11 September, America has been awash with security threats, but this one is reported to have caught the government's attention because the details are unusually specific.
The White House says it believes the threats are credible, and security at nuclear facilities has been stepped up.
The information is said to concern plans by Islamic militants to attack the Three Mile Island nuclear plant in Pennsylvania.
The White House is still examining the reports and has so far decided against issuing a formal public warning.
Trucker power
As part of America's continuing fight against terror, up to three million lorry drivers across America are being enlisted to keep a close watch on bridges and tunnels, because of fears that trucks carrying hazardous materials could be used as weapons.
Drivers will be offered training on detecting suspicious activities and a toll-free telephone number is being set up for them to report anything which appears to be a threat, says the BBC's Alex Van Wel in Washington.
Since 11 September haulage companies have discouraged their drivers from talking about routes or cargo and asked them to keep their doors locked to reduce the chances of hijacking.
A CIA official recently told a Senate committee that attackers were more likely to deliver chemical or biological weapons through the use of trucks, ships or planes rather than using missiles.