In just nine days, the attention of Americans will turn at last to national politics. It has been a warm summer in Philadelphia and Los Angeles, the cities hosting the Democratic and Republican Party conventions. It has also been so far a rather lacklustre campaign between the two presumed candidates, Vice-President Al Gore and Texas Governor George W. Bush, neither of whom possesses an abundance of charisma.

But beginning on July 31st in Philadelphia, and then August 14th in Los Angeles, the nation's political parties and candidates will strut their stuff for four days. The next US president will reach the widest audience to date and thus define himself and his policies before the American people, both in convention speeches and in his choice of a vice-presidential running mate. The television networks will broadcast live at least an hour each evening, and CNN will offer hours of nightly live coverage.

And that's why there are problems already.

The political conventions offer the opportunity of visibility and history. It was on July 15th, 1960, in Los Angeles that the Democratic Party nominee, John F. Kennedy, issued a call to 80,000 Democrats in the Coliseum and 35 million watching on television: "I believe the times demand invention, innovation, imagination, decision. I am asking each of you to be pioneers on that New Frontier. My call is to the young in heart, regardless of age, to the stout in spirit, regardless of party - to all who respond to the scriptural call `Be strong and of good courage; be not afraid, neither be thou dismayed'."

Such moments can define eras, and cities, protest groups and politicians crave their moment in the limelight. Seduced by the opportunity of promotion, both cities are striving to present themselves well to a national audience.

But this is not 1960, and on the heels of the disruptive protests at the recent World Trade Organisation conference in Seattle, and the World Bank meeting in Washington, critics are saying that both cities' police departments are acting brutally and repressively in the pre-clean-up and spruce-up campaigns. Moreover, they contend that police departments in the US are generally out of control.

Philadelphia's image suffered on July 13th in a brutal incident caught on videotape and aired repeatedly on both US television and the BBC.

On that day, just before 1 p.m., police spotted a green 1999 Chevrolet being driven erratically in a residential neighbourhood in north Philadelphia. A check revealed the car to be stolen and police began a pursuit.

The suspect driving the car was a black man named Thomas Jones (30), a car mechanic with a long arrest record who was clearly resistant to the idea of being detained again. A local television station happened to have a helicopter in the area, which spotted the chase and began broadcasting live.

After Mr Jones crashed the car, he fled on foot but was captured by police, who began to beat him. But one of the officers left the engine of his patrol car running, with the keys in the ignition. Mr Jones jumped in and sped away, but not before police began shooting at the car in a barrage of bullets, shattering the rear windscreen. Some 43 shell casings were found.

Mr Jones, it was later learned, was hit five times, but kept driving for 18 blocks until hemmed in by other police cars.

At that point, the officers dragged the wounded man from the car. The videotape shows 10 officers surrounding Mr Jones, who was now lying on the ground. All 10 beat him with batons and kicked him for 28 seconds. One officer hit him in the shins with a walkie-talkie. Altogether, 59 blows were inflicted. Four of the officers were black and six were white.

"Clearly, the videotape doesn't look good, any fool will admit that," said Police Commissioner John Timoney.

The issue is not one of race, say critics, acknowledging that some of the officers were black, or even of Philadelphia's police department. The problem is that Philadelphia is not an aberration but a common example of an accepted police brutality that is rampant all across the US.

The beating of Mr Jones is "part of an epidemic going on around this country", declared Congressman John Conyers of Michigan, a ranking member of the House Judiciary Committee. New York Congressman Greg Meeks added: "I'm distressed that Philadelphia has again joined the city of New York, the city of Chicago, the city of Los Angeles. So you see we have the same old problem, just a different zip code."

Philadelphia's Congressman Chaka Fattah said: "The use of force in an uncontrolled way for all the world to see brings to our city attention that is not desired."

When officers use excessive force, they usually go unpunished, even when there are videotapes. A Los Angeles Times investigation into 18 recent cases of police brutality caught on video showed that the officers rarely face criminal charges. From a 1998 Florida case where two Polish tourists videotaped a policeman beating a mentally ill man to a case last week in Lawrenceville, Georgia, that showed police beating a drunken-driving suspect as he lay on the ground, few cases end up in court.

If any place is sensitive to the community outrage that can follow such incidents of police over-reaction it is Los Angeles, however, where a riot that took 50 lives occurred after the 1993 beating of drunken-driving suspect Rodney King.

In an attempt to limit the kind of street protests that plagued Seattle, and citing concerns over the security of both delegates and police, the Los Angeles Police Department decided to keep demonstrations miles away from the Democratic Convention site downtown. Several of the 24 groups registered to protest, on matters ranging from the death penalty to the environment, filed a lawsuit.

Federal Judge Gary Feess agreed with them, ruling that the LAPD was attempting to stifle First Amendment rights to free speech.

"There is going to have to be an accommodation to allow the plaintiffs to reach their intended audience," said the judge. The protesters can't be shunted off to the side. "I don't believe the First Amendment will tolerate that."

Angry at the court, and declaring that the ruling will hinder convention security, the LAPD says it will obey the judge.

With police feeling defeated and misunderstood, protesters angry, and inner-city tensions running high, the political parties will have a challenge on their hands this summer to persuade Americans that another New Frontier is at hand.