Anti-corporate activists who triggered a national protest movement by occupying a square near Wall Street could not be more focused, except on one little detail: what exactly it is that they want.

After three weeks of permanent demonstration at Zuccotti Park in Manhattan, protesters have astonished doubters - and won attention from both President Barack Obama and his Republican opponents - with their staying power, organisational savvy and ability to grow.

The core sleeping rough is only a few hundred, but more than 5,000 people streamed through rush-hour traffic last week and smaller branches of the Occupy Wall Street brand are popping up in cities nationwide to protest the fate of the little guy in America, or what they call 'the 99 per cent'.

They use every latest social media tool. They rake in thousands of dollars in donations to buy basic supplies for the camp. By and large they maintain steely discipline when marching so as to avoid provoking the police.

They are in every way the model of a grassroots protest movement - except for that absence of a unified demand.

On Saturday, the start of the fourth week for a protest that initially seemed unlikely to survive a few days, about 1,000 people swarmed away from New York's Financial District to a park further uptown.

The gathering showcased the strengths of the movement. Peaceful, brimming with intellectual discussions about economic policy, and unfazed by the scorching sun, the crowd took part in what the protesters call their General Assembly - a daily gathering where practical matters are discussed.

Because megaphones would contravene the laws on unsanctioned demonstrations, the protesters use an ingenious method of relaying what speakers have said in a chorus through the crowd.

For an hour, the demonstrators discussed minutia of how to maintain and improve their protest machine. Only no one was saying where the machine wanted to go.

A forest of handmade cardboard protest signs decried everything from the Afghanistan war to the bailout of Wall Street institutions and soaring college tuition fees.

But the absence of a simple, common goal frustrated some.

"The problem with their street theater is that it discredits the whole thing. It doesn't seem serious," said writer Chris Gay, 53.

"They have got the attention, but now what? Once you have people's attention, you need some proposals."

Scathing criticism was leveled on Friday by none other than Liberia's peace campaigner Leymah Gbowee, shortly after she was named joint winner of the Nobel Peace Prize.

She praised the US activists for their energy, but said: "If you are doing a protest you need to have an agenda. If you wake up in the morning and poke a guitar, take a drum downtown and someone is singing and another one is dancing and movie stars are coming and saying do this, do that... and everyone is confused, you'll be there for a long time."

Some pundits believe that a likely avenue for Occupy Wall Street is to develop into a left-wing version of the conservative Tea Party movement.

The Tea Party has likewise lacked unity over much other than anger at the status quo. Yet it proved a powerful weapon during the 2010 congressional elections.

According to this scenario, the youth-driven activists of Occupy Wall Street would then reignite Obama's tattered base just in time for what will likely be a bitter reelection fight in 2012.

A budding alliance between the activists and high-profile trade unions already has the potential to generate considerable street power.

Yet few protesters seem ardent about Obama, the Democrats or politicians in general. And they seem highly unlikely to seek the equivalent of the potent relationship that the Tea Party forged with right-wing media and Republican billionaires.

"People realise we have to be careful. It is an election season, but the banks control the government," Lindsey Smith, 25, said, holding a placard that linked Wall Street's titans to Washington's masters.

"They give millions and millions of dollars to the politicians. Wall Street funds campaigns and then they get bailed out. It's completely corrupt."

Disillusioned with the political and business elite, the protest movement appears determined, for now, to instead follow its idiosyncratic path.

If anything, the lack of hurry to name a goal reinforces the message that these protesters don't play by normal rules.

"The exhausted political machines and their PR slicks are already seeking leaders to elevate, messages to claim, talking points to move on. They, more than anyone, will attempt to seize and shape this movement," the latest edition of the protest camp newspaper The Occupied Wall Street Journal said.

"For Wall Street and Washington, the demand is not on them to give us something that isn't theirs to give. It's ours. It's on us. We aren't going anywhere. We just got here."

At Saturday's rally, Neha Kagal, 26, a student from India, said the protest's diffuse nature is 'the good part of it'.

"That's what gets more people involved - and that's what revolutions are made of."