Two men divided by politics came together on Sunday morning to commemorate the 10th anniversary of the terrorist attacks on the twin towers of the World Trade Centre in New York and the Pentagon outside Washington.

US President Barack Obama, a Democrat, and his predecessor, George W. Bush, a Republican, gathered with their wives at Ground Zero in Lower Manhattan, the site where hijackers plunged two aircraft into the World Trade Centre, felling the twin towers.

At a ceremony in New York, family members read the names of those killed in the attacks. The city observed a moment of silence at 8.46 am, the time at which hijackers crashed American Airlines Flight 11 into the North Tower of the World Trade Centre. Bells tolled in churches across the city.

A second hijacked plane hit the South Tower of the World Trade Centre at 9.03 a.m. The third attack, on the Pentagon, occurred at 9.37 a.m. and at 10.03 a.m., a fourth hijacked plane was forced down in a field in Shanksville, Pennsylvania, by passengers who fought hijackers after learning of the other attacks. Nearly 3,000 people were killed in the attacks on 9/11.

At a solemn ceremony, Obama read from the Bible. He chose Psalm 46, which talks about God as “our refuge and strength, a very present help in trouble.”

Bush, who was the President at the time of the attacks, read a letter sent by Abraham Lincoln to a woman who lost five sons in the Civil War. “I feel how weak and fruitless must be any word of mine which should attempt to beguile you from the grief of a loss so overwhelming,” Bush read, quoting Lincoln. “But I cannot refrain from tendering you the consolation that may be found in the thanks of the republic they died to save.”

In his weekly radio address on Saturday, Obama said: “We have taken the fight to Al-Qaida like never before.” He said the US was stronger 10 years after the attacks. “As a resilient nation, we will carry on,” he told Americans.

Al-Qaida’s leader Osama bin Laden was killed by US commandos in an operation at Abbottabad, Pakistan, in May. However, on Sunday, New York and Washington were on high alert as top US officials warned that Al-Qaida was plotting an attack to coincide with the 10th anniversary of 9/11.

In New York, the police set up checkpoints at bridges and tunnels and beefed up their presence at the Grand Central train station and Times Square.

At the ceremony in New York, the city’s mayor, Michael Bloomberg, said of those who died in the attacks: “They were our neighbours, our friends, our wives, children and parents.” He said a “perfect blue sky” had turned into “the blackest of nights” on 9/11.

At the Pentagon, across the Potomac River from Washington, a moment of silence was observed at the time American Airlines Flight 77 crashed into the west side of the seemingly impregnable headquarters of the US military. Defence Secretary Leon E Panetta told the crowd that “no words can ease the pain you still feel”.

On Saturday, Vice-President Joseph Biden attended a memorial service at Shanksville, Pennsylvania, where hijackers crashed another plane.

“We’re here today to remember and honour 40 men and women who gave their lives so others could live theirs-decent, honourable women and men who never imagined 10 years ago tomorrow that when they said goodbye to their children, when they kissed their loved ones goodbye and walked through that door, that they were doing it for the very last time,” Biden said.

Biden was joined by former President Bill Clinton and George W Bush.