WASHINGTON - A planned weekend dedication of the Martin Luther King Jr. Memorial on the National Mall has been postponed until September or October as approaching Hurricane Irene dashed hopes of paying tribute to the late civil rights activist on the 48th anniversary of his "I Have a Dream" speech, organizers said Thursday.

Executive architect Ed Jackson Jr. told The Associated Press that the hurricane bearing down on the East Coast had forced the postponement of the dedication originally planned for 11 a.m. Sunday. President Barack Obama was to have been one of the scheduled speakers beside the King sculpture erected on a 4-acre (1.6-hectare) site in the nation's capital.

Harry Johnson, the president of the foundation that built the memorial, said at a subsequent news conference that he decided Thursday afternoon to postpone the dedication after studying forecasts that suggested Irene's passage had the potential of making weather conditions unsafe for visitors amid the threat of high winds and rains. He also said a Saturday black-tie gala event had to be postponed.

"We all are saddened by this. I remained optimistic all day, but Mother Nature is Mother Nature," Johnson said at a news conference. But he added, "The memorial is going to be there forever."

The forecasts threatened heavy winds and rains in Washington as Irene was expected to take an unpredictable path up the East Coast this weekend, the weather service said.

Organizers previously had said they had expected up to 250,000 people for the dedication and celebration of the King legacy on the mall. The memorial was to have been dedicated on the 48th anniversary of King's famous speech delivered less than 1 mile (1.6 kilometre) away on the steps of the Lincoln Memorial.

The hurricane was the second disruption for organizers who also had to contend with a rare East Coast earthquake on Tuesday.

The 5.8-magnitude earthquake that caused a crack on the upper part of the nearby Washington Monument had forced organizers to change a venue for a Saturday service anticipating the memorial's dedication. The interfaith service had been planned Saturday the National Cathedral, but that landmark building suffered damage from falling capstones from the quake centred in neighbouring Virginia.

Located between monuments to Presidents Abraham Lincoln and Thomas Jefferson, King's memorial is the first monument on the National Mall honouring a black leader. The memorial is a 30-foot-tall (9-meter-tall) sculpture in which King appears to emerge from granite carved to resemble the sides of a mountain.

The memorial faces southeast across the Tidal Basin from the Jefferson Memorial and was sculpted by Chinese artist Lei Yixin.