The incident on Tuesday night occurred as about 1000 protesters with the Occupy movement, which campaigns for economic and social equality, were demonstrating outside the home and office of the US president.
The Secret Service sent a robot to check out the device thrown on to the White House lawn, the Fox News cable news channel reported.
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No arrests were reported.
A reporter covering President Barack Obama and wife Michelle's celebration of her 48th birthday said the press corps was barred from leaving the White House for 45 minutes upon the couple's return from a nearby restaurant.
Several hundred protesters affiliated with the Occupy Wall Street movement had converged on the West Lawn of the US Capitol earlier on Tuesday to decry the influence of corporate money in politics and voice myriad other grievances.
Organisers had touted the rally, known as Occupy Congress, as the largest national gathering of Occupy protesters to date and secured a permit that would have allowed up to 10,000 people to participate. By mid-afternoon, the protest appeared to have fallen far short of those goals.
Still, participants said they were optimistic about the strength of the Occupy movement, which began in September when protesters pitched tents in a lower Manhattan park. The movement has since spread to dozens of cities, including Washington. While many cities have moved to evict the protesters, the National Park Service has allowed encampments to remain in two public squares near the White House.
"I'm encouraged," said Jon Wynn, 63, of North Carolina, who travelled to Washington to attend the protest and visit friends.
"There's energy here, even if there's not a whole lot of people."
The Occupy movement includes activists who want to change government from within and anarchists who oppose all government. Tension between the two camps was evident at Tuesday's gathering, where some taunted police while others participated in earnest group discussions about how to influence their elected representatives.
The protest comes amid numerous polls that show 84 per cent of Americans disapprove of the job Congress is doing, near an all-time low.
While the rally was mostly peaceful, there were some scuffles between police and protesters along walkways leading to the Capitol. By mid-afternoon Tuesday, four people had been arrested, US Capitol police said, one for allegedly assaulting a police officer and three accused of crossing a police line.
Later on Tuesday, small groups of protesters entered House office buildings in a bid to meet with individual members of congress.
Participants also planned to march to the Supreme Court and the White House.
