"I apologise to my supporters for emotions that they were exposed to during the elections," said Mr Kwasniewski, an ex-Communist, upon returning from a holiday in Spain.
It was not clear exactly which emotions he was referring to, but many Poles were outraged at the news that Mr Kwasniewski lied about being a university graduate. Thousands have taken to the streets in protest, demanding that presidential elections be annulled.
Mr Kwasniewski, 41, defeated President Lech Walesa by 3.4 per cent, or about 650,000 votes, in the 19 November runoff elections. He is to take office on 23 December. "I will take the presidential oath with a clear conscience," he said.
On Saturday, the Supreme Court ruled the election was valid but said Mr Kwasniewski had broken electoral law by falsely claiming he held an economics degree. The court rejected an application, backed by almost 600,000 Walesa supporters, that the victory be annulled, saying it was impossible to determine how Mr Kwasniewski's claims had affected people's preferences.
"This is obviously not a comfortable situation," said the Prime Minister, Jozef Oleksy, Mr Kwasniewski's colleague in Poland's last Communist government. "It has cast a shadow over the whole campaign."