Action on Junk Food Marketing analysed 750 adverts shown during the X Factor on ITV and the Simpsons and Hollyoaks on Channel 4 over 20 hours.
It found one in 10 promoted fast food restaurants, confectionery or supermarket 'junk food'.
But the government said advertising was not to blame for childhood obesity.
The analysis, which was carried out by researchers at the University of Liverpool, found that unhealthy food items accounted for 11% of all adverts and around half of all food adverts.
They said the most frequently shown adverts for unhealthy food products came from supermarkets such as Aldi and Morrisons, followed by fast food chains such as Dominos and Kentucky Fried Chicken (KFC).
Chocolate manufacturers like Lindt and Cadbury and brands such as Clover and Flora Buttery were also included on the "unhealthy" ads list.
The researchers looked at adverts shown during 10 hours of X Factor programmes and another 10 hours of early evening Channel 4 programmes during the run-up to Christmas 2013.
The campaign group Action on Junk Food Marketing, whose members include the Children's Food Campaign and the British Heart Foundation, said children's TV viewing peaks around 20:00 but laws to protect children from targeted advertising only cover children's programmes, which tend to be broadcast earlier in the day.
Simon Gillespie, chief executive of the British Heart Foundation, said: "Parents don't expect their children to be bombarded with ads for unhealthy food during primetime TV, but that's exactly what happens.
"Even when the show is over, junk food marketers could be reaching out to young people online. A lack of regulation means companies are free to lure kids into playing games and entering competitions - all with a view to pushing their product."
Prof Mitch Blair, officer for health promotion at the Royal College of Paediatrics and Child Health, said advertising junk food during family-friendly programmes like the X Factor is known to work.
"Not only are children and young people easily influenced and parents worn down by pester power, but food companies wouldn't spend huge amounts of money if it wasn't effective."
Campaigners want the ban on "junk food" advertising to be extended to 21:00.
"Children should not be commercially exploited and the advertising industry must take some responsibility for helping tackle the growing problem of childhood obesity," Prof Blair said.
The government said advertising was just one aspect in determining children's choice of food and one part of the package aimed at tackling childhood obesity and poor diet. It added that it was keeping "this area under review".
The Advertising Association went further saying the report was "lobbying dressed up as science" and the current rules on advertising were working.
Communications director Ian Barber added: "The UK's evidenced-based approach to the advertising rules works, balancing sensible protections with the freedom to advertise, allowing companies to compete - to the benefit of us all - and providing important funding for free-to-air TV."
But there is general agreement that with around one-third of UK children now overweight or obese, encouraging families and children to eat healthier diets is important.
Dr Alison Tedstone, director of diet and obesity at Public Health England, said they were working with the food industry to promote healthier products on TV through their Change4Life campaign.
"We recognise that we are all influenced by food adverts on TV. We are all eating too many calories and too much salt, fat and sugar which impacts on our health, causing obesity which increases our risk of cardiovascular disease, type-2 diabetes, and some cancers."
Children are being exposed to TV adverts promoting unhealthy food which should be banned during primetime family viewing, says a campaign group.