BLACK smoke hung above the eastern French city of Strasbourg yesterday as a summit of Nato leaders agreed short-term backing for the alliance’s military presence in Afghanistan but was overshadowed by an orgy of burning and looting.

The decision dashed Barack Obama’s hopes of European support for his military surge in Afghanistan and amounted to the minimum possible commitment of extra troops by European leaders.

Nine countries agreed to send up to 5,000 extra troops, 900 from Britain, but only during the Afghan elections this summer – far short of the permanent commitment that Washington and London had hoped for. The French president, Nicolas Sarkozy, refused to send any more troops at all.

Demonstrators clad in black attacked a police and customs station on a bridge linking France and Germany that had served only hours earlier as the backdrop for a show of unity by Nato leaders led by Obama.

Protesters stormed a hotel, setting it alight and pilfering alcohol from the bar, according to witnesses. Michelle Obama, the US first lady, and Carla Bruni-Sarkozy, her French counterpart, were forced to cancel a visit to a cancer hospital because of the upheaval.

Police fought running battles with protesters, making dozens of arrests as the 28 leaders held a “summit on the Rhine” intended to revive an alliance built to counter the Soviet Union. Despite the trouble, the talks, held on the 60th anniversary of Nato, ended in a flurry of mutual congratulation as Angela Merkel, the chancellor of Germany, and Sarkozy hailed the event as another success after Thursday’s G20 summit in London.

Anders Fogh Rasmussen, the Danish prime minister, was appointed Nato’s next secretary-general despite Turkish objections. He called the alliance “the most successful peace movement the world has ever known”.

Signalling the resurrection of the traditional Franco-German partnership, Sarkozy said he and Merkel had been determined the meeting would produce concrete results. “The age when international summits achieved nothing is over,” Sarkozy said, hailing the return of France to Nato’s command structure after a 43-year absence.

Even so, Obama, who has presented himself as the leader of an America that can no longer go it alone, had reason to feel disappointed. America’s Nato allies, including Britain, Germany, Spain, Portugal, Italy, Turkey, Croatia, Greece, Poland and the Netherlands, agreed to send additional forces, but not the permanent fighting force he had envisaged.

Robert Gibbs, the White House spokesman, said Washington was satisfied with this and Obama heralded “concrete commitments” from Europe. It emerged, however, that the troops are to be deployed mainly on training missions rather than in a combat role.

Gordon Brown presented the deal as a coup, saying it showed that other European countries were beginning to accept the need to do more to support the British and US effort. “We have persuaded many other countries today about the importance of this election period. We have got some evidence of burden-sharing,” he said.

He reiterated his call for longer-term help, saying it would be to “everyone’s benefit” if other European countries examined their resources with a view to doing more. “We are continuing to press our allies,” he said.

Nato’s ability to succeed in Afghanistan against the Taliban is seen as a crucial test of the power and relevance of an alliance that was built for the cold war. There is growing scepticism in Europe about whether more troops will bring stability to a country devastated by decades of war.

Strasbourg itself seemed a war zone yesterday as thousands of police stood nervously on street corners behind barriers while sirens wailed and helicopters rumbled overhead.

Many residents had left for the weekend after boarding up their homes and businesses. The spouses of the Nato leaders enjoyed a tour of the city’s 11th-century cathedral but had to return to lodgings later in the day as security began to unravel. Even before the talks began, several dozen protesters had attempted to break through the security cordon but were repulsed with tear gas.

Later, police were attacked by several hundred demonstrators with rocks, catapults and firebombs. The demonstrators then stormed the bridge linking France with Germany, where they set fire to a customs house and smashed the windows of television trucks.

On the German side of the river, about 5,000 demonstrators had gathered peacefully in the hope of crossing into Strasbourg but were held back by police backed with water cannon. “No nations, no border, fight law and order,” they chanted.

The alliance also seemed sure to arouse hostility in the Muslim world by choosing Rasmussen as its secretary-general. Tayyip Erdogan, the Turkish prime minister, had criticised the appointment because of Rasmussen’s handling of the uproar over a Danish newspaper’s publication of the prophet Muhammad cartoons in 2005.

“Every head of state and government is fully convinced that Anders Fogh Rasmussen is the best choice for Nato,” said Jaap de Hoop Scheffer, the alliance’s outgoing secretary-general.

“A solution has been found also for the concerns expressed by Turkey and we are unanimous in this,” he said.

As the summit opened on Friday, Obama had promised to repair damaged relations with Europe and asked for support for his new strategy, which will add 21,000 American troops to a force of 38,000 struggling against the resurgent Taliban.

A senior US official travelling with Obama said yesterday that the administration expected that pledges and commitments from other Nato nations would come in over the next few weeks. Obama and the allies also endorsed a return to normal relations with Russia, nine months after Moscow invaded Georgia.

The mayhem in the streets marked the return of protests at international summits after a lull for most of the past decade. Clashes at a G8 summit in Genoa in 2001 resulted in a protester being shot dead by police but the terror attacks on September 11 seemed to change the climate, making it harder to engage in protest politics.

The Italian government, remembering the trouble in Genoa, has decided to stage the G8 summit this year on the island of La Maddalena off the coast of Sardinia. It is accessible only by boat.

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