The United States is stronger while al-Qaeda is on the road to defeat 10 years after the 9/11 attacks on the country, President Barack Obama has said.

Obama was speaking ahead of planned events to mark the 10th anniversary of the attacks on New York, Washington, DC and Pennsylvania, and amid a security crackdown following “specific and credible” threats against New York City and Washington on the eve of the anniversary.

“Thanks to the tireless efforts of our military personnel and our intelligence, law enforcement and homeland security professionals, there should be no doubt: today, America is stronger and al-Qaeda is on the path to defeat,” Obama said in a weekly radio and internet address on Saturday.

The president also referred to the killing of Osama bin Laden, the former leader of al-Qaeda, asserting that popular protest movements in the Arab world were a testament to the lack of influence al-Qaeda has over the region’s people.

“We’ve forged new partnerships with nations around the world to meet the global challenges that no nation can face alone,” Obama said. “And across the Middle East and North Africa, a new generation of citizens is showing that the future belongs to those that want to build, not destroy.”

Obama spoke as New York police put out a show of force, setting up checkpoints at major traffic intersections and at key locations on the city’s subway system.

Al Jazeera’s Asad Hashim, reporting from the city’s financial district, said the impact of the tight security on Saturday was markedly less than Friday due to the start of the weekend.

The NYPD’s response came after US intelligence reported the existence of specific, credible but uncorroborated threats involving a car or truck bomb plot linked to the anniversary.

The intelligence emanated from a source in the Afghanistan and Pakistan region, who told US officials that three men had been sent to the US with the training and intention of setting up car bombs in the two cities, a senior counterterrorism official told Al Jazeera.

US officials said they had not been able to get another source to confirm the threat or any details, but that security will be ramped up in both cities as the anniversary approaches on Sunday.

Hillary Clinton, the US Secretary of State, had earlier indicated that the US believed that al-Qaeda was involved in the threat.

At least two of the suspects are believed to be US citizens, the Associated Press news agency reported.

At a meeting of his senior national security team on Sunday, Obama directed officials to “pursue vigorously all threat information and to ensure a heightened state of vigilance and preparedness” ahead of the anniversary.

US military bases overseas have been placed at the second-highest “bravo” alert level in response to elevated security risk.

The president also said that efforts must not let up in the days and weeks following the anniversary weekend.

Meanwhile, in Shanksville, Pennsylvania, US Vice-President Joe Biden and former Presidents George Bush and Bill Clinton were present at a sombre ceremony honouring the passengers and crew of Flight 93, which crashed in a field after being hijacked on September 11, 2001.

They were accompanied by relatives of the 40 victims of the crash. Speakers at the dedication of the $62m Flight 93 National Memorial also included Gordon Felt, the president of the Families of Flight 93 group. Felt’s brother Edward had been on board the flight, and had telephoned a friend from on board to report that the flight had been hijacked.

A white stone wall, bearing the names of those who struggled with the hijackers on the Boeing 757 airliner during the attack, was unveiled in the rural Pennsylvania field where the plane crashed.

Security was tight at the ceremony, and the US Federal Aviation Administration declared a no-fly zone up to 5,500m over the site, which is about 200km west of Washington, DC, for most of Sunday, when further ceremonies will be held.

Obama visited the Arlington National Cemetery on Saturday with his wife, Michelle, to remember those US soldiers who have been killed in the Iraq and Afghanistan wars. They visited Section 60 of the cemetery, a burial ground for service members killed in those two wars, which have so far claimed more than 6,213 US soldiers’ lives.

The Obamas were due to participate in a service project on Saturday afternoon in the Washington, DC area, ahead of ceremonies on Sunday where the president is scheduled to visit the New York City, Shanksville, and Pentagon sites where hijacked planes crashed 10 years ago.

He will be joined in New York by former President Bush. The memorial at the World Trade Center site in New York City will begin shortly before 8.30am local time, with moments of silence to be observed for each of the times that the four planes crashed, and the two times when the World Trade Center towers fell.

Speakers at the event will read out the names of the almost 3,000 people who were killed in the attacks that day. The ceremony will also mark the opening of two memorial pools at the World Trade Center site, which sits at the former footprints of the two buildings, to the public.

Obama closed the day by speaking at a memorial event at the Kennedy Center in Washington, DC.