The rally took place on Monday night, Polish state news agency PAP reported.
It was co-organised by the Ukrainian group Euromaidan Warszawa and Poland's Committee for the Defence of Democracy (KOD), among others, under the motto Stop Russian terror.
Protesters gathered outside the Warsaw office of the EU's executive Commission, calling on the bloc to provide Ukraine with "essential defensive weapons," such as F-16 fighter jets, drones and air defences, to repel Russian attacks.
They also appealed to the EU to help isolate Russia on the international stage and use "frozen Russian assets to support Ukraine."
Ukraine's victory against the Russian invasion "is crucial not only for their own people but also for the entire democratic world," Euromaidan Warszawa said.
KOD also called for the EU to overcome Hungary's veto against the EUR 50 billion financial aid package for Ukraine, and to "admit Ukraine to the bloc as soon as possible," so that there would be funds for the reconstruction of the country.
Protesters handed EU officials a letter urging the bloc to "take immediate action" in support of Ukraine.
Monday is day 684 of Russia’s war on Ukraine.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
Source: PAP, wnp.pl, Euromaidan Warszawa