The investigation will focus on whether criminal charges should be brought for "public propagating of fascism" during march on Poland's Independence Day.
Prosecutors in Poland opened an investigation on Monday to determine if statements expressed during a march by far-right nationalists in Warsaw this month violated laws against propagating racism.
The march held on November 11, Poland's Independence Day, drew an estimated 60,000 participants. Many marchers carried Poland's national flags, while some had flags with Celtic crosses, a white supremacist symbol, or banners with slogans like "White Europe of brotherly nations."
Warsaw prosecutors spokesperson Magdalena Sowa said that the investigation would focus on whether criminal charges should be brought for the "public propagating of fascism and calls for hatred," offences punishable by up two years in prison.
The march's organisers and the people who held provocative banners are the focus of the investigation, she said.
Poland's President Andrzej Duda and European Union lawmakers have condemned the event.
Other members of the Polish government have praised it as a manifestation of patriotism, raising concerns among some Poles that the ruling Law and Justice party might lend the far-right some legitimacy.
Jewish group concerned
Also on Monday, a Jewish group in Poland said that it had raised its "indignation" at the racist slogans in a meeting with ruling party leader Jaroslaw Kaczynski the day after the march.
The Jewish community of Poland issued a statement saying its leader, Leszlaw Piszewski, and the country's chief rabbi, Michael Schudrich, had a "frank and open conversation" of more than an hour with Kaczynski.
The statement said that when the Jewish leaders expressed their concerns, "Kaczynski stated that he was strongly opposed to these slogans as well."
Also discussed at the meeting was security for the Jewish community and the protection of a Jewish cemetery in the city of Kalisz, according to the group's statement. It said Kaczynski promised to show interest in the case.
Anti-Semitic and anti-Muslim slogans
The annual march, which has become one of the largest far-right events in Europe, also saw the use of anti-Semitic and anti-Muslim slogans and chants this year.
One large banner read "Deus Vult" in Gothic lettering. Latin for "God wills it," it was a cry used during the First Crusade in the 11th century, when a Christian army from Europe slaughtered Jews and Muslims in Palestine and around.
In recent years, it has been used by the radical right to show hostility to Islam.