Over 2 billion people watched the 2008 Olympic Opening Ceremony on TV

"Based on television audience data collected from 38 markets around the world,  The Nielsen Company estimates that just over 2 billion people - almost one third of the world’s population - watched the Beijing Olympics Opening Ceremony.
The Nielsen Company drew the data from 38 key markets in all regions around the world including host nation China, the United States, Brazil, South Africa, Italy and Australia. Viewing level varied across regions and markets, impacted by factors such as time zone and broadcast time differences.
Comparing regions, the highest audience reach was in Asia-Pacific, where more than five in 10 people watched the Opening Ceremony, followed by Europe (30%) and North America (24%). Looking at individual markets, the percentage of people tuning into the Opening Ceremony varied widely – from the expected high of host nation China, followed by South Korea (where 44% of people watched), Greece (43%) and Australia – all recent hosts of the Summer Olympics (Seoul 1988, Sydney 2000 and Athens 2004). Lower numbers of people tuning into the Opening Ceremony were reported in markets such as Indonesia (8%) and Argentina (11%). Viewing levels were also impressive in the United States, where it is estimated that 65 million people tuned into the broadcast."

Source:  Press release from Nielsen, 14th August 2008

Methodology:  "Estimates are based on data sourced across 38 markets from The Nielsen Company, AGB Nielsen Media Research, BBM Nielsen Media Research, Finnpanel, IBOPE, TAM India and OzTAM. 2 of 2"

Note - This is thought to be the largest ever TV audience - unless you know different!  For example it seems to have been bigger than the 2011 Royal Wedding