What they were not prepared for was the post-prandial aggression of oil traders who kicked and punched them back on to the pavement.

“We bit off more than we could chew. They were just Cockney barrow boy spivs. Total thugs,” one protester said, rubbing his bruised skull. “I’ve never seen anyone less amenable to listening to our point of view.”

Another said: “I took on a Texan Swat team at Esso last year and they were angels compared with this lot.” Behind him, on the balcony of the pub opposite the IPE, a bleary-eyed trader, pint in hand, yelled: “Sod off, Swampy.”

Greenpeace had hoped to paralyse oil trading at the exchange in the City near Tower Bridge on the day that the Kyoto Protocol came into force. “The Kyoto Protocol has modest aims to improve the climate and we need huge aims,” a spokesman said.

Protesters conceded that mounting the operation after lunch may not have been the best plan. “The violence was instant,” Jon Beresford, 39, an electrical engineer from Nottingham, said.

“They grabbed us and started kicking and punching. Then when we were on the floor they tried to push huge filing cabinets on top of us to crush us.” When a trader left the building shortly before 2pm, using a security swipe card, a protester dropped some coins on the floor and, as he bent down to pick them up, put his boot in the door to keep it open.

Two minutes later, three Greenpeace vans pulled up and another 30 protesters leapt out and were let in by the others.

They made their way to the trading floor, blowing whistles and sounding fog horns, encountering little resistance from security guards. Rape alarms were tied to helium balloons to float to the ceiling and create noise out of reach. The IPE conducts “open outcry” trading where deals are shouted across the pit. By making so much noise, the protesters hoped to paralyse trading.

But they were set upon by traders, most of whom were under the age of 25. “They were kicking and punching men and women indiscriminately,” a photographer said. “It was really ugly, but Greenpeace did not fight back.”

Mr Beresford said: “They followed the guys into the lobby and kept kicking and punching them there. They literally kicked them on to the pavement.”

Last night Greenpeace said two protesters were in hospital, one with a suspected broken jaw, the other with concussion.

A spokeswoman from IPE said the trading floor reopened at 3.10pm. “The floor was invaded by a small group of protesters,” she said. “Open outcry trading was suspended but electronic trading carried on.”

Eighteen police vans and six police cars surrounded the exchange and at least 27 protesters were arrested. A small band blocked the entrance to the building for the rest of the evening.

Richard Ward, IPE’s chief executive, said that the exchange would review security but denied that protesters had reached the trading floor. However, traders, protesters and press photographers confirmed to The Times that the trading floor had been breached.

Mr Ward would not discuss whether he would press charges, and said he would not know until this morning if there had been any financial loss.

Greenpeace later started a second protest at the annual dinner of the Institute of Petroleum at the Grosvenor House Hotel on Park Lane, in Central London. Greenpeace claimed that five campaigners had got into the Great Hall. About 30 protesters were outside the hotel, some blocking the front entrance by sitting down and locking themselves together, while others sounded klaxons and alarms. Climbers scaled scaffolding to unfurl a banner reading, “Climate change kills, oil industry parties”.

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