The American Embassy in Cairo helped a young dissident attend a US-sponsored summit for activists in New York, while working to keep his identity secret from Egyptian state police.

On his return to Cairo in December 2008, the activist told US diplomats that an alliance of opposition groups had drawn up a plan to overthrow President Hosni Mubarak and install a democratic government in 2011.

The secret document in full

He has already been arrested by Egyptian security in connection with the demonstrations and his identity is being protected by The Daily Telegraph.

The crisis in Egypt follows the toppling of Tunisian president Zine al-Abedine Ben Ali, who fled the country after widespread protests forced him from office.

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The disclosures, contained in previously secret US diplomatic dispatches released by the WikiLeaks website, show American officials pressed the Egyptian government to release other dissidents who had been detained by the police.

Mr Mubarak, facing the biggest challenge to his authority in his 31 years in power, ordered the army on to the streets of Cairo yesterday as rioting erupted across Egypt.

Tens of thousands of anti-government protesters took to the streets in open defiance of a curfew. An explosion rocked the centre of Cairo as thousands defied orders to return to their homes. As the violence escalated, flames could be seen near the headquarters of the governing National Democratic Party.

Police fired rubber bullets and used tear gas and water cannon in an attempt to disperse the crowds.

At least five people were killed in Cairo alone yesterday and 870 injured, several with bullet wounds. Mohamed ElBaradei, the pro-reform leader and Nobel Peace Prize winner, was placed under house arrest after returning to Egypt to join the dissidents. Riots also took place in Suez, Alexandria and other major cities across the country.

William Hague, the Foreign Secretary, urged the Egyptian government to heed the “legitimate demands of protesters”. Hillary Clinton, the US Secretary of State, said she was “deeply concerned about the use of force” to quell the protests.

In an interview for the American news channel CNN, to be broadcast tomorrow, David Cameron said: “I think what we need is reform in Egypt. I mean, we support reform and progress in the greater strengthening of the democracy and civil rights and the rule of law.”

The US government has previously been a supporter of Mr Mubarak’s regime. But the leaked documents show the extent to which America was offering support to pro-democracy activists in Egypt while publicly praising Mr Mubarak as an important ally in the Middle East.

In a secret diplomatic dispatch, sent on December 30 2008, Margaret Scobey, the US Ambassador to Cairo, recorded that opposition groups had allegedly drawn up secret plans for “regime change” to take place before elections, scheduled for September this year.

The memo, which Ambassador Scobey sent to the US Secretary of State in Washington DC, was marked “confidential” and headed: “April 6 activist on his US visit and regime change in Egypt.”

It said the activist claimed “several opposition forces” had “agreed to support an unwritten plan for a transition to a parliamentary democracy, involving a weakened presidency and an empowered prime minister and parliament, before the scheduled 2011 presidential elections”. The embassy’s source said the plan was “so sensitive it cannot be written down”.

Ambassador Scobey questioned whether such an “unrealistic” plot could work, or ever even existed. However, the documents showed that the activist had been approached by US diplomats and received extensive support for his pro-democracy campaign from officials in Washington. The embassy helped the campaigner attend a “summit” for youth activists in New York, which was organised by the US State Department.

Cairo embassy officials warned Washington that the activist’s identity must be kept secret because he could face “retribution” when he returned to Egypt. He had already allegedly been tortured for three days by Egyptian state security after he was arrested for taking part in a protest some years earlier.

The protests in Egypt are being driven by the April 6 youth movement, a group on Facebook that has attracted mainly young and educated members opposed to Mr Mubarak. The group has about 70,000 members and uses social networking sites to orchestrate protests and report on their activities.

The documents released by WikiLeaks reveal US Embassy officials were in regular contact with the activist throughout 2008 and 2009, considering him one of their most reliable sources for information about human rights abuses.