Polish anti-LGBT protesters clashed with riot police in the south-eastern city of Lublin on Saturday just before a gay pride march that has highlighted increased social tensions in the devoutly Catholic nation ahead of an election next month.

Nobody was hurt in the clashes but police said they had detained at least 30 people. Right-wing groups in Poland have expressed strong opposition to gay rights and have targeted such marches in the past.

The nationalist ruling Law and Justice (PiS) party has made "LGBT ideology" a key issue in its campaign ahead of the parliamentary elections on 13 October, saying it is an invasive foreign influence that undermines traditional Polish values.

"We've had death threats, (this violence) was about forcing us not to have this march," Bartosz Staszewki, organiser of Saturday's march, told Reuters.

As a helicopter circled overhead, television footage showed, riot police backed up a by water cannon separating the marchers, who brandished the rainbow flag of the Lesbian, Gay, Bisexual and Transgender community, from the protesters who had tried to block their route.

Protesters tried to pelt the marchers with eggs, private broadcaster TVN 24 reported.

Many pride marches have become battlegrounds, with riot police often having to protect marchers against counter-protesters. The mayor of Lublin had tried unsuccessfully to have Saturday's march banned, citing security concerns.

On Friday the Court of Appeal upheld the district court's decision to allow the march to go ahead.

Activists and researchers say that the rhetoric used by the ruling party is homophobic and that its goal is to mobilise PiS's conservative base.

PiS leader Jaroslaw Kaczynski has publicly urged Poles to resist the "travelling theatre" of Pride marches which he described as "a real threat to... the Polish state".

PiS officials have previously said they are not against gay couples, they just want them to exist solely in private. They counter charges of homophobia by saying they are responding to Polish public opinion, which remains conservative on the issue.

A survey conducted in August by a global market research firm Ipsos for a Polish website OKO press found that the LGBT movement was ranked by Poles as one of the most serious threats to the country, second only to climate change.

PiS has maintained a comfortable lead in polls for months with support of over 40%, according to a majority of recent polls.