Venezuelan opposition leader María Corina Machado is calling for elections to be held next year.
During a news conference in Caracas on Thursday, the Nobel Peace Prize winner said the country's move toward democracy was "irreversible".
She also thanked the Trump administration for opening the "floodgate to freedom" by capturing her opponent, former President Nicolás Maduro.
The same day, Venezuela's interim president, Delcy Rodríguez, said elections would be held and that her government should oversee a "transition of this magnitude" - which Machado dismissed as illogical. Rodríguez, who was Maduro's vice-president, did not set a date for voting.
Machado said there would be elections in 2027, but also did not offer an exact timeline.
The transition in Venezuela would be "irreversible, organised, pacific", Machado said standing before a slogan which read "Let Us Be Free".
Her return to the country comes after she repeatedly made earlier efforts to re-enter Venezuela, and failed.
Last year Machado left Venezuela on a speedboat and in darkness to travel to Norway and accept the Nobel Peace Prize. Soon after, in January, the US seized Maduro.
The Trump administration then backed Rodríguez as leader. She was sworn in as interim president and later signed an oil deal that granted favourable terms to the US.
Although it remains unclear what Machado plans to do next, she has previously said she would run for president again if an election were held. Her ability to return to her country was contingent on approval from the Trump administration.
Speaking about her return, Machado said she had heard one word repeated by the thousands of supporters who thronged her route into the capital - freedom.
"I come to carry out a task and complete a mandate," she said.
The opposition leader said she had not received a message or invitation to meet from Rodríguez, and would not personally participate in election negotiations between opposition representatives and the government.
But she wished them well in their work, she said, in order to see the electoral body reformed so that every Venezuelan, inside the country and abroad, could vote.
Shortly before Machado's news conference, Rodríguez delivered a speech in a conciliatory tone, saying she had "instructed the security agencies that no one be detained without judicial order, except in cases of flagrant commission of crimes".
Rodríguez also spoke of "the doors being open" in Venezuela to returning exiles and talked of dialogue and building consensus.
