JALALABAD, Afghanistan -- Gunfire rattled through the streets and smoke billowed into the sky while Afghan troops fought Taliban militants on Thursday after an audacious assault against a police station that killed 10 officers and a university student in one of Afghanistan's largest cities.

All seven attackers are killed, but they made their point. Government forces face a huge challenge in securing upcoming elections in what will be a major test of their abilities as foreign troops wind down their combat mission.

The brazen assault in Jalalabad, an economic hub near the border with Pakistan, shows the Taliban are following through on their threat to use violence to the disrupt April 5 vote, which will be the first democratic transfer of power since the 2001 U.S.-led invasion that ousted the Islamic militant movement. President Hamid Karzai is constitutionally barred from seeking a third term.

It began before dawn when a suicide bomber blew up his explosives-laden car outside the police station, located near the palatial residence of Nangarhar provincial governor Attaullah Ludin.

Six gunmen then rushed into the station as two more bombs exploded nearby -- one hidden in a motorized rickshaw and another in a vegetable cart.

That prompted a fierce battle that lasted more than four hours, with Afghan police and soldiers chasing gunmen down the street. All seven attackers were killed, deputy Interior Minister Gen. Mohammad Ayub Salangi said. Associated Press photos show security forces grabbing each arm and dragging a lifeless body of one of the attackers from the scene with armoured vehicles parked nearby.

Police said the 11 killed included a city district police chief and a university student caught in the crossfire.

The Interior Ministry said 15 policemen also were wounded in the attack, which it said was aimed at sabotaging the upcoming elections.

Taliban spokesman Zabihullah Mujahid claimed responsibility for the assault, saying the attackers wore suicide vests and that nearly 30 policemen were killed. The Islamic militant group frequently exaggerates casualty figures.

The nearby state-run Afghan radio and television building was badly damaged with windows shattered in the initial suicide bombing.

The Taliban have carried out numerous attacks in Jalalabad, Kabul and elsewhere in the east. But the choice of a police station as a target reflected an effort to show they can still penetrate heavily secured areas despite numerous U.S. and Afghan offensives against them in recent years.

The elections include ballots for provincial elections, but the most closely watched is the presidential race. Karzai's successor will guide the country for the next five years as most U.S. and allied forces leave the country by the end of 2014.

As part of the withdrawal process, Afghan authorities on Thursday released dozens of prisoners who had been held by foreign troops, including some 40 who had been detained by British forces in southern Helmand province, officials said.

The freeing of prisoners detained by international troops from the Parwan Detention Center has strained relations between Washington and Karzai, particularly in the wake of the Afghan leader's increasingly anti-American rhetoric and refusal to sign a long-negotiated bilateral security deal that would allow thousands of American and allied troops to stay in the country beyond the end of a 2014 deadline to leave.

The U.S. military has said some of those set free were directly linked to attacks that have killed or wounded dozens of U.S. or coalition personnel, as well as Afghan security forces and civilians.

The British Embassy had a more cautious response.

Embassy spokesman Maj. Tim James says Britain disagreed with Thursday's release but "has to respect it because it is an Afghan decision."

James declined to discuss specific charges facing the men, saying only that "we believed that we had enough evidence against these individuals to merit them going into the Afghan judicial system."

James said 77 were released -- of which 39 were former British detainees.

Abdul Shakoor Dadras, a member of the Afghan review panel, said 55 were released Thursday, including 40 who had been held by British forces. Dadras said the men were released for a lack of evidence and that 48 other cases have yet to be decided.

The European Union, meanwhile, announced that a mission to observe the elections was underway. The team consists of 16 international experts from 13 European countries. It said locally based European diplomats will also participate on election day and during the tallying of votes.

Fears of fraud are high after the disputed 2009 election, which was won by Karzai. Abdullah Abdullah, who was running in second place at the time but quit before the runoff vote because of allegations of vote-rigging, is one of the front-runners in the current race.

Abdullah's supporters said they hoped to reverse past mistakes during a campaign rally in his stronghold of Parwan province, northwest of the capital of Kabul.

"Dr. Abdullah is the person to build roads, schools and bridges," said Abdul Sattar, 65, as he sat in chairs set up in a sandy field for the rally. "If the main decision falls to God, he will win."


Gamel reported from Kabul. Associated Press writers Rahim Faiez and Amir Shah contributed to this report.