Polish President Andrzej Duda has called the promotion of LGBT rights an "ideology" more destructive than communism, in a campaign speech.
He is an ally of the ruling nationalist Law and Justice Party (PiS), and is seeking re-election on 28 June.
He said his parents' generation had struggled against communist ideology for 40 years and "they didn't fight for this so that a new ideology would appear that is even more destructive".
Critics say PiS has an anti-gay agenda.
The LGBT rights group ILGA-Europe says Poland is the worst-performing country in the EU in terms of LGBT rights, in an index published last month.
PiS won a majority in parliament with a conservative-nationalist agenda strong on Catholic values, including support for traditional families and opposition to gay marriage.
Speaking to supporters in Brzeg, southwestern Poland, Mr Duda said "parents are responsible for the sexual education of their children," and "it is not possible for any institutions to interfere in the way parents raise their children".
On 10 June he signed a "Family Charter" of election proposals, including pledges to prevent gay couples from marrying or adopting children and to ban teaching about LGBT issues in schools.
Tensions with EU
Putting LGBT rights activism in the same category as communism can be seen as inflammatory in Poland, where the anti-communist Solidarity movement led the struggle for democracy in the 1980s.
Many Poles agree with the PiS message that communism was a foreign ideology imposed on Poles by the Soviet Union.
PiS has clashed with the EU over judicial reforms which, according to PiS, are necessary to eliminate vestiges of communist-era corruption. Critics in the EU say PiS is politicising the judiciary and violating EU principles.
The European Commission has written to the heads of five Polish provinces expressing concern about resolutions in which they declare themselves "free from LGBT ideology". The EU's executive has reminded them of their duty to guarantee non-discrimination as a core EU value.
Opinion polls suggest that Mr Duda is favourite to win the 28 June election, though his lead has shrunk amid the economic impact of the coronavirus crisis.
One of his chief rivals, liberal Warsaw Mayor Rafal Trzaskowski of the centre-right Civic Platform (PO) party, has been criticised by religious conservatives for allowing discussion of LGBT issues in Warsaw schools. The election is likely to go to a second-round runoff vote.
In his speech in Brzeg, Mr Duda said "this is not why my parents' generation for 40 years struggled to expel communist ideology from schools, so that it could not be foisted on children, could not brainwash and indoctrinate them...
"They did not fight so that we would now accept that another ideology, even more destructive to man, would come along, an ideology which under the clichés of respect and tolerance hides deep intolerance".
Rival candidate Robert Biedron of the Left party - an LGBT rights activist - called Mr Duda's Family Charter "a radical document which divides Polish society, introducing standards reminiscent of the most brutal... times of Polish and European history".