Work to transform around 250 acres (101.2 hectares) of former industrial land into new green space at the heart of the Olympic Park, east London, is now underway.

The Olympic Delivery Authority (ODA) is planting 4,000 new semi-mature trees and 300,000 wetland parts as part of the project, which will provide the largest urban park to be created in the UK for more than a century. A large wetland bowl to the north of the Olympic Park site will contain the new reed beds, which will provide a space for visitors to watch the action on live screens before creating a tranquil riverside area in legacy mode.

Rainwater will be captured by the bowl through porous paving and will be "cleansed" by ponds, reedbeds and swales before flowing back into the river, which will also protect nearby properties from flooding. The London 2012 Garden will stretch for 0.5 miles (0.8km) between the Olympic Stadium and Aquatics Centre to include picnic lawns, seating and a range of plants from 250 different species.

A Royal Horticultural Society Great British Garden will be created overlooking the 2012 stadium, which will be designed by two amateur gardens - Hannah Clegg and Rachel Read - chosen by public vote. Hargreaves Associates, Tim O'Hare Associates, Bam Nuttall, Skanska, Palmstead Nurseries and Arup Landscape are among the wide number of companies working on the scheme.

ODA chair John Armitt said: "The parklands will be the centre-piece of the Olympic Park during the Games and are at the heart of the long-term transformation of this part of east London. "We have cleaned up formerly industrial land, much of it contaminated, and opened up inaccessible river banks to start creating a new great park that will be enjoyed by people and wildlife for generations to come."

Sport and Olympics minister Hugh Robertson added: "After the Games this former industrial waste land will be a wonderful community facility where people inspired by the Olympic and Paralympic Games will be able to go to play sport." Meanwhile, London mayor Boris Johnson has told the BBC that he is lobbying the International Olympic Committee (IOC) to stage the 2012 marathon and triathlon events at weekends, rather than during the week.

Johnson believes holding the events on a weekday would be disruptive, although IOC co-ordinator Denis Oswald said efforts were being made to reach "the best solution". Image: Dave Poultney/ODA