Catholic and veterans' groups in Poland are protesting against a concert by Madonna because it falls on the anniversary of the Warsaw Uprising.
Some are urging ticket-holders to boycott Wednesday's show, which comes 68 years after the city's failed revolt against Nazi occupation began.
Organisers of the concert have agreed to show a short clip about the events of 1944 before the performance.
Every year, Poles commemorate the 200,000 lives lost during the uprising.
One Catholic group called Krucjata Mlodych, or Youth Crusade, has started an online campaign urging people not to attend the concert.
They say more than 50,000 people have signed up to their Don't Go To See Madonna campaign.
The group also says anti-Madonna Mass services and street prayer sessions have been held.
They accuse the singer of offending their faith through her use of burning cross and crown of thorns imagery, and say she promotes pornography and sexual deviation.
Billboards around the capital promoting the concert have been defaced with the sign of the Polish Home Army, the largest underground army in Nazi-occupied Europe, reports the BBC's Adam Easton in Warsaw.
Every year, at 1700 on 1 August, sirens wail across Warsaw and people stand still to pay their respects to the victims of the 63-day uprising, our correspondent says.
Conservative opposition MP Stanislaw Pieta has appealed to the government not to allow the concert to go ahead in Warsaw's National Stadium, Polish Radio reports.
Concert organisers have agreed to a proposal by city officials to show a short film about the uprising in the stadium before the show, in an attempt to appease the protesters.
Ania Pietrzak, a spokeswoman for concert organiser Live Nation, told the Press Association: "It is an important moment in Polish history, so we have decided to remind people of that moment."
It is the latest controversy to hit the 53-year-old singer's MDNA tour.
In Paris, some fans booed her when she ended the show after only 45 minutes.
She also angered supporters of France's right-wing National Front party, by showing a swastika imposed on the face of the party's leader, Marine Le Pen.