Those who marched in Warsaw held up banners featuring such slogans as "Abortion is my right" and "Abortion with no borders."
Police in the Polish capital used tear gas in an attempt to disperse demonstrators who gathered at Roman Dmowski roundabout in the city centre, the PAP news agency reported.
Warsaw police said on Saturday they had detained six people over incidents including disturbing the peace and insulting officers in the previous evening.
Friday marked the third night of country-wide protests, held in defiance of a ban on large gatherings amid the COVID-19 pandemic.
People took to the streets in other cities on Friday evening, including Łódź, central Poland, Katowice, the capital city of the Silesian Region, and Wrocław in the west.
The demonstrations were sparked by a government announcement that a contentious ruling from the Constitutional Tribunal, restricting access to abortion, would come into legal force.
The court's detailed verdict was published in an official gazette on Wednesday night, a step necessary for it to take effect.
In October, the court ruled that termination of pregnancies due to foetal defects should be banned, eliminating the most common of the few legal grounds for abortion in the predominantly Catholic country.
Under the new rules, abortions will only be permitted in cases of rape and incest, and when the mother's life or health is endangered.