Polish President Bronislaw Komorowski on Sunday urged reconciliation at a ceremony in Ukraine marking the 70th anniversary of the massacre of tens of thousands of Poles by Ukrainian nationalists in World War II.
His conciliatory comments in the northwestern town of Lutsk were however overshadowed by a security breach after the ceremony when a young Ukrainian man walked up to him and slapped a raw egg on his shoulder. The president was not hurt in the incident.
While little known outside eastern Europe, the bloodshed in the Nazi-occupied Volhynia and Galicia regions in the west of modern Ukraine in July 1943 has strained ties between Ukraine and Poland.
Neither government wants history to cast a pall over their current relations, with Warsaw encouraging Ukraine to move away from Russian influence and realise its ambition of joining the European Union.
Komorowski, joined by Ukrainian Deputy Prime Minister Kostyantyn Gryshchenko, said the past should not endanger the countries' friendship.
"An honest interpretation of the past should serve reconciliation and cooperation between our peoples and our two independent states," said Komorowski.
"I hope that this wound between our brotherly nations heals quicker," he said.
Komorowski hinted that Moscow was the only winner when Poland and Ukraine feuded.
"We should not forget that only a third party -- that has always threatened our independence and freedom -- has won when the Ukrainian and Polish peoples have been in conflict."
For his part, Gryshchenko said both governments had a responsibility to overcome the past and build a shared European future.
"We have no reason to argue, nothing should separate our states. We have one aim, a common European future," he said.
The attacker, hiding an egg in his hand, walked up to Komorowski after the ceremony, bypassing security.
The man broke the egg on Komorowski's shoulder but the president appeared unfazed and swapped his soiled jacket for another.
The man -- a Ukrainian who has not been named -- has been detained by police. It was not clear what his motives were.
The Polish parliament in a resolution on Friday stopped short of calling the 1943 massacre a genocide -- a move that could have caused a diplomatic row -- and opted instead for the more cautious wording of "ethnic cleansing characterised by signs of genocide".
Most mainstream historians believe that tens of thousands of Poles were killed in Nazi-occupied Poland by nationalists from the Ukrainian Insurgent Army (UPA).
The UPA wanted to minimise the number of Poles in a future independent Ukrainian state, an ambition for which they waged guerilla warfare against Soviet forces into the 1950s.
At the time of the massacres, the northwest Volhynia region was part of Nazi-occupied Poland but became part of the Soviet republic of Ukraine after the war and then independent Ukraine in 1991.
The killings provoked bloody reprisals by Polish partisans grouped in the anti-Nazi and anti-Soviet Home Army (AK), who in turn killed thousands of Ukrainians.
The current Ukrainian government under President Viktor Yanukovych has shown considerably less enthusiasm about the wartime nationalists than his anti-Kremlin predecessor Viktor Yushchenko.
In a provocative move, Yushchenko shortly before leaving office in 2010 awarded wartime nationalist leader Stepan Bandera the posthumous honour of Hero of Ukraine. The award was revoked under Yanukovych.
But the issue divides Ukraine, which is split between the nationalist west and the more pro-Russian east.
Almost 150 deputies from Ukraine's ruling Regions Party and the Communist Party have issued a declaration to the Polish parliament calling on it to describe the killings as genocide.
Ukraine's first post-independence president Leonid Kravchuk blasted the petition which he said "could be equated with state treason", Ukrainian media said.