Poland held a minute's remembrance for the 200,000 victims of the failed Warsaw uprising against the Nazi occupation 60 years ago.
At 1700 local time, Warsaw came to a standstill to mark the exact hour the city's home army launched its rising.
They surrendered 63 days later, having had virtually no help from Poland's US, British and Russian allies.
Gerhard Schroeder was at the ceremony, becoming the first German chancellor to attend such an event.
His visit was seen as an important gesture in Poland, said the BBC's Adam Easton in Warsaw.
Relations between the two countries have been strained over German property claims and disagreements over the war in Iraq, our correspondent added.
"The Warsaw uprising is part of our continent's drive to freedom... and liberation from Nazism," said Mr Schroeder, who laid a wreath at Warsaw's monument to the uprising.
"I am honoured to have been invited here... This is a very noble gesture towards a country which has brought a lot of pain to Poland by the war it started," he added.
During 63 days of fighting in 1944, the Soviet army held back on the outskirts of Warsaw as leader Joseph Stalin did not want the resistance to lay the foundations of an independent post-war Poland.
When the rising was brutally suppressed, the SS carried out Hitler's orders to level the city and deport its inhabitants.
Western allies, the US and Britain, were blamed for not bringing more help to the city.
Britain, in particular, refused to airlift in Polish troops who had taken refuge in the UK and wanted to join their countrymen's struggle.
Polish PM Marek Belka - whose own father took part in the uprising - said an allies' apology would be welcomed by Poland.
He told BBC's Today programme on Saturday: "Maybe we should start with recognition for the very fact that the uprising in Warsaw was a huge effort of Polish people suffering under the Nazi occupation.
"I don't know if 'apology' is the right word, but we should put the history straight."
US Secretary of State Colin Powell, also in Warsaw for the commemorations, denied Poland had been betrayed by the allies.
"I don't think betrayal is the appropriate word here. There were difficulties in reaching Poland. Attempts were made to help but not enough help was available or forthcoming from the international community," he said.
"But the important thing now is that the US and Poland are united. Poland will never be alone again as long as the US is there to stand alongside Poland."
The UK's Europe Minister Denis MacShane, whose own father was among the Polish commandos unable to fly to the aid of the Polish resistance, said he was "not quite sure" an apology would be appropriate.
Sunday's ceremonies in Warsaw included a dramatic reconstruction of an armed barricade, at the exact moment the uprising started on 1 August 1944.
This was followed by a march and several wreath-laying ceremonies before a concert to honour the heroes of the uprising.