They are calling on Syria to withdraw its 15,000 soldiers from Lebanon and to demand a full investigation into the bombing.
The four-hour protest -- the largest since the end of Lebanon's civil war in 1990 -- unfolded peacefully under an intense military and police presence.
Some in the crowd yelled "Syria out!" and "We don't want a parliament that acts as a doorkeeper for the Syrians" as they made their way through the streets of Beirut.
"It is my civic duty as a Lebanese to take part in this uprising," Youssef Mukhtar, a 47-year-old engineer, told the Associated Press. "Enough bloodshed and disasters. It is the 21st century, and people should be able to govern themselves. The situation has become unbearable and we have to regain our country."
Syria, which has denied any connection to the attack that killed Hariri, indicated Monday it would consider a withdrawal of at least some of its troops in accordance with the Taif agreement, which ended Lebanon's 15-year civil war.
"President Bashar al-Assad stressed more than once in (our) talks his firm intention to press ahead with the implementation of the Taif agreement and to plan a Syrian withdrawal from Lebanon in line with this agreement," Amr Moussa, the head of the Arab League, told reporters after meeting Syrian President Assad in Damascus.
Despite the Syrian statements, American and European leaders continued to call for an immediate withdrawal of Assad's forces and a fuller investigation into the bombing.
"Just as the Syrian regime must take stronger action to stop those who support violence and subversion in Iraq and must end its support for terrorist groups seeking to destroy the hope of peace between Israelis and Palestinians, Syria must also end its occupation of Lebanon," President Bush said in a speech in Belgium.
"Our shared commitment to democratic progress is being tested in Lebanon, a once-thriving country that now suffers under the influence of an oppressive neighbor," he added.
Later Monday, Presidents Bush and Jacques Chirac of France issued a joint statement demanding a Lebanon "free from foreign domination."
"We urge full and immediate implementation of U.N. Security Council resolution 1559 in all its aspects, including its call for a sovereign, independent and democratic Lebanon as well as for the consolidation of security under the authority of a Lebanese government free from foreign domination," they said.
The increased diplomatic pressure drew a sharp response from Syrian officials who called the remarks "extremely dangerous."
"Syria has always been a positive and constructive partner in the region," Syrian Expatriates Minister Buthaina Shaaban told CNN. "What's happening today is extremely dangerous and they (the United States) have to change course and work with Syria as a partner."
Even as diplomatic debate continued over the presence of Syrian forces within Lebanon, the European Union joined the United States and others in calling for a full and independent investigation into the bombing that killed Hariri.
"The Council (of ministers) calls for an international investigation without delay to shed light on the circumstances and those responsible for this attack," said a communique approved by EU foreign ministers.