They held signs which read "Make Peace Great Again" and "Refugees welcome," among others.
The Summit between US President Donald Trump and Russian President Vladimir Putin is scheduled for July 15.
The rally, which advocated causes including human rights, democracy and environmental awareness overflowed with banners and flags denouncing Trump and his Russian counterpart.
Chairman of the Finnish Communist Party Juha-Pekka Vaisanen who was among protesters proposed a Finish 'anti-military' sauna ritual for the visiting politicians, saying, "We wipe out the militarism from them."
Helsinki isn't the only European location seeing Trump-related protests over the weekend; tens of thousands of people demonstrated against his visit to London on Friday, while several thousand came out to do the same in Scotland on Saturday, where Trump has spent much of the weekend playing golf.
Trump arrived in Finland from Scotland on Sunday.
