On October 22, the Constitutional Tribunal ruled that abortion due to foetal defects was unconstitutional, banning the most common of the few legal grounds for ending a pregnancy in the largely Catholic country.
Curbing access to abortion has been a long-standing ambition of the ruling Law and Justice (PiS) party, but it has stepped back from previous legislative proposals amid a widespread public backlash.
In Warsaw, the protest spilt across the city late into the evening, with some walking more than 10 kilometres (6.21 miles) from PiS leader Jaroslaw Kaczynski's house towards government buildings in the centre.
Polish media estimates put the Warsaw protest at 15,000 people, despite restrictions that limit public gathering at 10 in the capital.
After the ruling goes into effect, abortion will be banned in the case of foetal abnormalities and will be legal only in the case of rape, incest or a threat to the woman's health.
More protests are planned across Poland on Friday (October 30), including a mass gathering in Warsaw.