President Obama on Monday marked the 20th anniversary of the fall of the Berlin Wall in a video message broadcast before a huge crowd in Germany, calling the destruction of the wall a "rebuke of tyranny." 

Obama, who drew criticism for not personally appearing in Berlin for the celebration, expressed regret that he could not attend.  But he said the fall of the wall on this day in 1989 is something he will "never forget." 

"There could be no clearer rebuke of tyranny. There could be no stronger affirmation of freedom," Obama said. 

The video message was aired after Secretary of State Hillary Clinton, who led the U.S. delegation in Germany for the celebration, addressed the crowd. 

"History did not end the night the wall came down. It began anew," she said, praising Berliners for helping transform Europe. 

The Obama administration cited a scheduling conflict for the president's inability to go to Berlin. The White House said he simply did not have the time to go, with a trip to Asia scheduled for later in the week. 

Obama was scheduled to leave for Asia Wednesday, but has delayed leaving until Thursday so he can attend a memorial service for the victims of the Fort Hood shooting on Tuesday. 

Some critics saw Obama's decision not to travel to Berlin as a snub to Germany and Chancellor Angela Merkel. 

"Barack Is Too Busy," Germany's Der Spiegel magazine declared in a headline last month, writing that Obama had declined Merkel's invitation to attend the anniversary celebration. 

Obama has traveled to Germany since taking office, but he has traveled to Berlin -- the site of his major speech in July 2008 during his overseas campaign tour. 

On several of the president's overseas stops to date he has expressed regret for past American behavior, but the Berlin Wall anniversary was seen as an opportunity for the president to honor an American and Western victory for which the U.S. need feel no regret. 

"It is a true shame that the president of the United States -- this man who cloaks himself in the rhetoric of hope -- won't be pausing to remember," former House Speaker Newt Gingrich wrote in a column last week in The Washington Examiner.