Ukrainian police in the tense southern city of Odessa on Monday discovered grenades near the site of commemorations marking two years since clashes between Moscow and Kiev supporters killed 48.
Street battles between armed backers of the pro-Western government in Kiev and the Kremlin in the historic Black Sea port killed six people and culminated in a building inferno in which 42 pro-Russians died.
The violence erupted at the height of Ukraine's pro-Russian eastern insurgency and sparked fears in Kiev that Moscow was preparing an all-out invasion of the former Soviet state.
That assault never came but the bloodshed ratcheted up the tensions and Russia later accused Ukraine of covering up an investigation into who was ultimately responsible for the deaths.
The two-year separatist revolt Russia denies supporting has now killed more than 9,300 people and -- along with the annexation of Ukraine's strategic peninsula of Crimea -- plunged Moscow's relations with the West to a post-Cold War low.
Around 1,000 people carrying flowers and banners with pictures of the victims massed on Monday outside metal detectors set up around the square at which the worst carnage occurred.
"We will never forgive or forget," a group of several dozen pro-Russians chanted while waiting for police to allow them into the commemoration site.
Victims' relatives eventually sent black balloons and white doves into the bright blue sky as it became clear that the riot-geared officers would not let them approach the building in which their sons and daughters died from the flames and toxic fumes.
Several also plunged to their deaths after being unable to withstand the heat of the raging blaze -- made worse by the Molotov cocktails being hurled at the building by some far-right Ukrainians.
About 3,000 members of the security forces were called into Odessa as a precaution to prevent a repeat of the violence.
Those troops helped find three grenades hidden near Kulykove Pole -- the square where the Trade Union House in which the pro-Russians took refuge was torched.
Police also reported making a handful of arrests but no serious altercations between two groups with diametrically opposed views of who was to blame for what happened in 2014.
The Office of the UN High Commissioner for Human Rights said the fatal chain of events was sparked when a group of pro-Russians armed with baseball bats and guns "provoked" the participants of the pro-Ukraine rally.
A Ukrainian court is so far only looking into the case of 20 suspects accused of shooting dead six people during the pro-Kiev march.
The results of a probe into who was responsible for torching the building in which the pro-Russians took shelter when chased by armed Kiev supporters have not been submitted to court.
"I urge the government of Ukraine to follow up on the recommendations of the Council of Europe International Advisory Panel and to carry out an independent and transparent investigation," Kiev's EU Ambassador Jan Tombinski said in a statement marking the anniversary.
Moscow said Russian Foreign Minister Sergei Lavrov on Monday also stressed the need to bring the Odessa inquiry to a close during a telephone conversations with US Secretary of State John Kerry.