The crowd massed in Manara Square, a central traffic circle in the West Bank city, waving banners and shouting slogans against the American administration.
"Obama, you despicable man, we want self-determination!" shouted the demonstrators, many of them members of Palestinian president Mahmud Abbas's Fatah party.
Fatah called for the protest after the United States used the first veto of President Barack Obama's administration to nix a resolution that would have condemned continued Israeli settlement building.
The resolution, drafted by the Palestinian leadership in an attempt to pressure Israel to halt settlement construction in the West Bank and east Jerusalem, was supported by all 14 other members of the Security Council.
The United States said its veto should not be interpreted as support for Israeli settlement construction, but that it did not believe the United Nations was the best place to resolve the decades-old Israeli-Palestinian conflict.
But in Ramallah, the veto drew fury and disappointment.
"No to negotiations with Israeli settlements and American arrogance," one banner held by demonstrators read.
The United States relaunched direct talks between Israel and the Palestinians began in September 2009, after a hiatus of nearly two years.
But the negotiations ground to a halt shortly after they started, with the expiry of an Israeli moratorium on settlement building in the West Bank.
The Palestinians insist they will not negotiate while Israel builds on land they want for a future state.
Palestinian officials have said they will launch a new bid for UN condemnation of Israeli settlement building, bringing a resolution before the General Assembly.