Police said seven activists - who are protesting against the construction of controversial dams in Spain and India - came down voluntarily and were arrested for suspected criminal damage.

One had been arrested earlier before being able to climb onto the wheel.

But two protesters are reported to be still at the top of the wheel.

They have been reached by Police climbers and an aerial negotiation is underway.

The protestors used climbing equipment, including harnesses, to scale the wheel - officially known as the London Eye - on Monday morning.

The protest has been mounted by Basque environmentalists from the group Solidarios con Itoitz and activists from the Indian group, Narmada UK.

At one stage, a police helicopter hovered 50 metres from the wheel, while an officer urged the protestors to come down.

Two protesters remain at the top of the wheel armed with supplies for a long stay

A red hammock was set up at the top of the wheel and the demonstrators pledged to stay up all night.

An activist, who gave his name only as Nick, said: "We chose the Millennium Wheel because, like the dams, we don't think it serves any purpose."

He said the protesters planned to tour Europe with their protest.

A protester is dwarfed by both the wheel and a passing jet

The wheel, which affords spectacular views over the capital, only reached its final position last Monday.

Due to open on Millennium Eve, it is London's fourth-tallest structure and will stay in position for at least five years, with a capacity to carry 15,000 people a day.

The wheel's sponsor, British Airways, said it was looking into the security implications of the protest.

The demonstrators are opposing alleged environmental destruction caused by dam projects in the Basque Country in Spain and in the Narmada valley in the western Indian state of Gujarat.

The activists unfurled huge banners with the slogans: "Stop The Dams", "Free Narmada, Free Itoitz" and "Let the Rivers Run Free".

Their spokesman on the ground said the climb began at 0600 BST and the protesters had food, warm clothing and rain gear.

It took the protesters about half an hour to scale the wheel, using a maintenance ladder built into it.

Site managers said there was little they could have done to stop the protesters.

Tim Renwick, from project construction firm Mace, said security would be reviewed.

But he said: "Short of using dogs and barbed wire here, there is nothing we can do."

It took activists 30 minutes to climb the wheel

The Narmada dam project dates back 40 years and has caused protests since work began 15 years ago.

The brainchild of India's first Prime Minister Jawaharlal Nehru, environmental campaigners - backed by Booker Prize winner Arundhati Roy - say it will submerge forest farmland and disrupt downstream fisheries.

There is also anger over the displacement of up to 250,000 people.