Thousands of Ukrainians marched in Kiev on Monday, decrying as "capitulation" a mooted pullback of troops fighting Moscow-backed separatists in the east and calling for victory in the five-year war.
Monday marks the anniversary of the foundation of the Ukrainian Insurgent Army (UPA), a group of nationalists who fought against Soviet troops in World War II alongside Nazi forces, and are accused of slaughtering Poles and Jews.
Nationalist forces customarily hold the "UPA March" on this date, with many of the former fighters in the conflict in eastern Ukraine among the demonstrators, as well as supporters of nationalist organisation Svoboda.
October 14 has also been designated as Ukraine's armed forces day.
Protesters chanted "No to capitulation!", "Ukraine above all" and "Russian language today, Russian tanks tomorrow".
The gathering is not the first protest against a peace plan for eastern Ukraine that would include broader autonomy for the separatist territories supported by Russia.
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President Volodymyr Zelensky is gearing up for a EU-supported summit with Russian leader Vladimir Putin, the first in years, in an effort to revive a stalled peace process for the region.
Protesters however said they saw any summit with Putin as a loss for Ukraine.
"There won't be peace if we agree to these conditions, Putin's conditions," said one former fighter who gave only his first name Mykola.
"Already I feel that we are giving up our national interests," said writer Nataliya Tysovska.
"If we pull back our troops again there will be a giant 'grey' zone that will be occupied by someone else." "The current government is taking steps that could lead to the capitulation of Ukraine as a whole," Oleksiy Kaida, a deputy head of the Svoboda nationalist movement, told AFP.
"We have to get back our lands at any price: not just Donbass but also Crimea," annexed by Russia in 2014, he said.
Zelensky took over in May from president Petro Poroshenko, pledging to end the conflict, but his efforts for new talks with Russia have been criticised.
The conflict has become the worst East-West crisis since the end of the Cold War and claimed 13,000 lives.
Ukraine has seen a rise in nationalism since the ouster of Kremlin-backed leader Viktor Yanukovych in 2014 sparked Russia's takeover of Crimea.