The returns counted so far showed the Lavalas Family Party had won at least 14 of the 27 seats in the Senate. The remaining 13 seats still have not been fully tallied.
The elections held May 21 were intended to reinstall constitutional government in Haiti. The returns appeared to confirm the sweeping win by the Lavalas Family party that analysts had predicted.
The Lavalas party also won 10 of the 30 Chamber of Deputies seats counted so far, and its candidates were leading in the other 20 races.
The win was expected to give momentum to Aristide's likely presidential bid in November.
Results were not yet available for the hundreds of local positions also elected on May 21.
Opposition parties say the elections were unfair, and dozens of government opponents have been arrested since the vote.
President Rene Preval, an Aristide supporter, locked lawmakers out of Parliament in January 1999 after an 18-month standoff with the Struggling People's Organization. In March 1999, Preval appointed by decree a new premier and the provisional electoral council that organized the elections.
The United States on Saturday condemned the climate of intimidation that followed Haiti's elections, saying reported arrests of prominent opposition leaders were especially troubling.
"We condemn the violence and call upon the government of Haiti to use restraint and to take immediate measures to rectify these incidents," the embassy said in a statement.
Opposition parties say at least 34 of their members have been arrested since chaotic balloting for local and legislative posts May 21. Fifteen are members of the Struggling People's Organization and 19 are from the five-party Space for Concord coalition.
Seven of those arrested were candidates. They included Sen. Paul Denis, a former parliamentary majority spokesman accused of illegal possession of arms; and Jean Limongy, a Space for Concord lower house candidate accused of incitement to violence.
Jean-Bertrand Aristide, the mentor of current President Rene Preval, is favored to win Haiti's presidential election at the end of the year.
More than 2 million Haitian voters, an estimated 60 percent of the electorate, cast their ballots for thousands of local and legislative offices in violence-free elections marred by numerous irregularities.
The opposition has unanimously condemned the elections. Most have said they will boycott second-round balloting for legislative offices scheduled for June 25.
Most opposition parties accuse Preval and Aristide of planning a one-party, totalitarian state. Aristide spokesmen have denied implication in all violence. Aristide has called his adversaries "hard losers."
At least 15 people were slain in politically related killings in the two monthleading up to the balloting.
By MICHAEL NORTON