With only a third of the three thousand polling stations reporting results, Sirleaf has taken a 60 per cent share of the vote across 14 counties.
The former FIFA world footballer of the year denounced widespread fraud and irregularities in both the casting and counting of votes in Tuesday's polls.
His party has already lodged complaints with the African and European Unions and the United States.
UN observers say they are not aware of any reports of disruptions or violations during election procedures.
Clutching a stack of ballots, George Weah showed each ballot was marked with a black thumbprint in the box for Mrs Sirleaf and stamp on the back with the National Elections Commission official stamp.
"While we were preaching transparency and democracy, our election was
fraudulent," Mr Weah said.
The controversy and allegations of fraud in Tuesday’s poll has aroused partisan anger among Mr Weah’s legion of supporters, sparking fears that violence could erupt on the streets before results are announced.
His supporters roared with approval as Mr Weah addressed them from the bandstand, vowing to continue his fight.
“I believe in you,” he said and was answered immediately by chants of “No Weah, no peace”