The ‘topping out' ceremony marked the completion of the £350m property's external structure.
Park Plaza Westminster Bridge hotel is London's largest ever new-build hotel and is set to open in spring 2010.
The hotel will offer 1,021 rooms and 2,700sqm of events space.
The 16-storey property is located on the South Bank and will have views of the Houses of Parliament and Big Ben.
Mayor of London Boris Johnson said: "This impressive development has removed an awful eyesore that has blighted this corner of the capital for too long. A sparkling new hotel is just what the South Bank needs. I am sure it will act as a major springboard for the regeneration plans for Waterloo Station and the surrounding area, on top of creating hundreds of jobs and boosting the local economy."
Park Plaza Hotels president and chief executive Boris Ivesha said: "We are passionate about London and are on track to deliver 5,000 beds across the capital, establishing Park Plaza Hotels as one of the city's most important hotel operators."
Park Plaza Hotels developments:
Park Plaza Marrakech - opening 2009
Art'otel Marrakech - opening 2009
Art'otel Cologne - opening 2009
Park Plaza Nuremberg - opening 2010
Art'otel Amsterdam - opening 2010
Park Plaza Westminster Bridge London - opening 2010
Art'otel Amsterdam - opening 2011
Art'otel London Hoxton - opening 2012