Ex-football star George Weah and Vice-President Joseph Boakai are the main contenders in the race to succeed her.
Liberia, founded by freed US slaves in the 19th Century, has not had a smooth transfer of power in 73 years.
Ms Sirleaf urged people to vote peacefully in a nation still recovering from a 14-year civil war.
"Your vote is about you and your family - not party, ethnicity," she said in an address to to the nation.
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A total of 20 presidential candidates are running to succeed Ms Sirleaf.
They include Alex Cummings, a former Coca-Cola executive, and MacDella Cooper, an ex-model and girlfriend of Mr Weah.
Ms Sirleaf, 78, is stepping down at the end of her two terms.
She took office in 2006, after her predecessor, Charles Taylor, was forced out of office by rebels in 2003.
Taylor is currently serving a 50-year prison sentence in the UK for war crimes related to the conflict in neighbouring Sierra Leone.
Mr Weah, 51, has chosen Taylor's ex-wife Jewel Howard Taylor as his running mate.
This is the former footballer of the year's third attempt to become president.
Ms Sirleaf has failed to campaign for Mr Boakai, fuelling speculation that the two have fallen out.
Hundreds of Liberians wait in a queue that snakes from the gates of St Theresa convent, through South Beach slum almost reaching the shoreline behind. Some elder members have brought plastic chairs to sit on - it's going to be a long day.
Charles Daniel, 70 , has been waiting since 06:30 GMT. "We want a good leader, not a ruler, he said. Men and women carrying large buckets full of boiled eggs and other snacks are here hoping to make a killing today.
Similar scenes can be found at hundreds of polling stations in and around the capital this morning as more than two million registered voters prepare to elect their new president. The streets of the capital are almost empty, everybody seems to be waiting patiently in line to cast their ballot.
Almost 2.2 million people are registered to vote in the election.
Parliamentary elections are also being held at the same time.
Campaigning has been dominated by promises to tackle poverty, corruption, and guarantee stability in the West African state.
Liberia is one of the poorest countries in the world, with more than 50% of people living in poverty.
It was one of three West African states badly hit by the 2014-15 Ebola outbreak which left more than 11,300 people dead.
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