U.S. President Barack Obama has canceled his trip to Poland because of volcanic ash over Europe, White House spokesman Robert Gibbs said on Saturday.
Kaczynski, his wife Maria, and a delegation of senior officials died in a plane crash near Smolensk in western Russia on April 10.
World leaders were due to arrive in Poland on Sunday to pay tribute to the late couple.
All air traffic has been suspended in several north European countries because of volcanic ash spewed out by the volcano, and the deputy chief of the Chancellery of the President of Poland, Jacek Sasin, said on Friday that some world leaders could be prevented from coming to pay their last respects to Kaczynski.
However, the Polish TVN24 channel said the country's authorities had no plans to postpone the ceremony.
Tens of thousands gathered in Warsaw earlier on Saturday in memory of Polish President Lech Kaczynski and 95 others killed in the air crash.
"We will always remember them," Prime Minister Donald Tusk told a crowd of some 100,000 people in the center of the capital, Polish radio reported.
WASHINGTON, April 17 (RIA Novosti)
"We are standing here to say goodbye to all those who perished in the crash," said acting president Bronislaw Komorowski.