The US secretary of State has said Syria's involvement in the affairs of its neighbours in the Middle East is blocking change across the region.
Ms Rice said Syria must withdraw troops from Lebanon
Condoleezza Rice urged the country to take responsibility for its actions, warning it was endangering peace.
She accused Syria of standing in the way of Lebanese, Iraqi and Palestinian aspirations.
Meanwhile, Syria's president has said that troops could be withdrawn from Lebanon within the next few months.
"We are starting to see a picture of a Syria that really is a blockage to a different kind of Middle East," Ms Rice said.
She called for fair elections in Lebanon, where the Syrian-backed government resigned on Monday following two weeks of protests in the capital, Beirut.
"When you think about what Syria is doing in Lebanon - you have Lebanese in the streets telling the Syrians to go home so that Lebanon can control its own future," she said.
Military intervention begins in 1976
30,000 troops in Lebanon during 1980s, currently 15,000
Syrian forces crucial in ending the Lebanese civil war in 1990 and maintaining peace
Calls for departure of the Syrians increase gradually with Israeli withdrawal in 2000
UN resolution calling for withdrawal of all foreign forces passed in Sept 2004
She said the US would be applying pressure to Syria to withdraw both its military troops and security services from Lebanon.
The country's President, Bashar al-Assad, told Time magazine that troops could be withdrawn within the next few months.
But he stressed this would depend on the security situation in Lebanon and Syria's own security concerns being met.
"It [withdrawal] should be very soon and maybe in the next few months. Not after that," Mr Assad said in an interview published on the magazine's website on Tuesday.
"The security in Lebanon is much better than before. They have an army, they have a state, they have institutions," he said.
"The second thing, which is related to Syria, is that after withdrawing we have to protect our border. We need to talk about our borders, because when Israel invaded in 1982, they reached that point. It was very close to Damascus."
Ms Rice - speaking after talks in London aimed to show world support for the Palestinian Authority - called for Lebanon to be free from "contaminating influences".
She said there was "a long list of concerns about a Syria that is standing in the way of Lebanese, Iraqis, Palestinians and others in their aspirations for a better world".
"The territory of Syria [is] being used to support an insurgency in Iraq, that is clearly standing in the way of the Iraqi people, who voted overwhelmingly for that better life [...] and in the Palestinian Territories you have Syria continuing to support Palestinian rejectionist groups."
The Lebanese cabinet resigned on Monday, two weeks after the killing of the country's former Prime Minister, Rafik Hariri.
Both the outgoing government and the Syrian government have been accused of involvement in the 14 February assassination of Hariri - charges they deny.
The opposition in Lebanon is expected to decide later on Wednesday whether it will participate in talks with President Emile Lahoud before he chooses a new prime minister.
The opposition leader, Walid Jumblatt, has already called on President Lahoud to resign as well.