ABC News obtained videos the White House is showing on Capitol Hill. The president says it is proof the Syrian government used nerve gas on its own people. It shows the after effects of the gas.
Obama will address the nation Tuesday, one day before a vote in the Senate to authorize use of force against Syria's regime.
On Saturday, thousands around the world protested against the proposed airstrikes on Syria. At least 150 protesters picketed the sidewalk in front of the White House and marched to Capitol Hill, chanting slogans such as "They say more war; we say no war" and carrying signs that said a war on Syria would be "Built on a Lie."
The demonstrators said their picket line is one that Congress shouldn't cross as it prepares to vote on the issue.
Medea Benjamin, a founder of the anti-war group Code Pink, said "There is a grass-roots uprising against the Democrats and the Republicans."
Many lawmakers in both parties oppose Obama's request for Congress to authorize using military force against Syria for a deadly Aug. 21 chemical gas attack the Obama administration blames on President Bashar Assad.
The message at a small noontime rally at Justin Herman Plaza in San Francisco was clear, with protesters chanting "U.S. NATO, hands off Syria!" and "Start a war but you can't feed the poor, can't take it no more."
These people oppose the idea of U.S. military airstrikes against Syria. An hour later the rally grew into a large protest march up Market Street.
"This is not what the American people want," protester Ann Domacour said. "We need diplomacy, need to get everyone out.
"I feel like people have forgotten the last 10 years," protester Jeremy Gameros said. "What's going on?"
This march made its way to the United Nations Plaza. In the meantime there was another rally going on in Union Square with a much different message.
While chanting "Stop the killing stop the hate," Syrian Americans urged the U.S. to stop the use of chemical weapons by the Syrian government against innocent civilians.
"The Assad regime is like cancer, the cancer is growing so we need to cut it out, we want airstrikes, not asking for boots on the ground," said Feras Alhlou with the Syrian American Council.
One protester dressed as House Democratic leader Nancy Pelosi, outside an event in the Mission where the real Congresswoman was giving a speech. The demonstrator got into the event and tried to upstage Pelosi with a sign, but she was escorted out by police.
Pelosi supports the president's use of airstrikes in Syria.
"Stopping the use of weapons of mass destruction is the pillar of our nation," she said. "The Assad regime uses them, that can't continue."
Pelosi knows the move has opposition on the streets where anti-war cries are expected to get louder in the days and weeks to come.
(The Associated Press contributed to this report)