The White House has called a demonstration against the Iran nuclear deal a "pro-war rally."
The main speaker at the demonstration, Republican presidential candidate Ted Cruz, is one of several GOP candidates who have said that, as president, they would be ready to launch an immediate military strike on Iran.
The rally was held outside the White House itself.
When asked about the demonstration, White House press secretary Josh Earnest said Cruz was holding a "pro-war rally," although he declined to say that all the demonstrators were pro-war.
At the rally, Cruz said the United States would become the "leading state sponsor and financier of radical Islamic terrorists" under the deal because it would lift economic sanctions on Iran in exchange for curbs on Iran's nuclear activities.
Opponents say such sanctions relief would usher in a flood of money that Tehran would use to bolster its military ventures in the Middle East.
At the rally, Cruz was interrupted by antiwar Code Pink activists, with whom he had a spirited debate over the deal.
The demonstration in Washington follows a July 22 protest by thousands against the deal in New York's Time Square.