Campaigners are using social media to help reunite missing children with their families.
The Big Tweet for Missing Children will enlist the support of millions of Twitter users in a day of action.
Every 30 minutes today a different child's appeal and photograph will be posted on Twitter using the hashtag #bigtweet
TV personalities including Stephen Fry, are lending their support to the effort to circulate the information about the missing children.
Last year 58,000 retweets resulted in at least two missing children being found and reunited with their families.
The Child Exploitation and Online Protection Centre (CEOP) which issues six to eight alerts each year, hopes a million people will sign up though its website to receive them.
An extension to the Child Rescue Alert system has also become operational, using text and email to send messages about missing children whose lives are believed to be at immediate risk.
The new service has been welcomed by Coral Jones, the mother of April Jones whose abduction from her home town of Machynlleth, Powys, in October 2012 triggered one of the alerts.
"You don't know what's around the corner, it could be your child next," she said. "I think everyone should sign up because you never know what will happen where you are."
It is estimated that 140,000 children and young people go missing in the UK every year according to the charity Missing People.
Kevin Gosden, from Doncaster, whose son Andrew went missing in 2007 aged 14 and has never been seen since, is encouraging people to support both campaigns, even if it does not result in him finding Andrew.
"If it does something to raise awareness of this issue, and it gets families to think about that and talk with their kids to be sure that they know the number for Missing People - 116 000 - then I think that also is a good thing," he said.
Anyone with any information about a missing child should call Missing People on 116 000.