Men's 100m final and other athletics seats are being snapped up by people who previously missed out on London 2012 Olympics tickets.
A million applicants who failed in the first buying round are able to apply for up to four tickets for one session between now and 23:00 on 17 May.
Events covered also include hockey, tennis, beach volleyball and football.
Some 900,000 tickets were made available on Friday, initially to 20,000 applicants.
They were available to people who were unsuccessful in two previous rounds, but who had initially thought they had secured tickets, only to be told those seats had already been sold.
The latest sale is for people not allocated any tickets in the initial application phase, who did not apply in the second round of sales.
Some 70,000 general tickets for the Olympic Park will be made available to this group from 17 May.
Games organisers announced just two hours before the sale began that athletics tickets would be available.
BBC sports news correspondent James Pearce said: "Presumably they had kept quiet for fear of demand being too great for a website which has sometimes struggled to cope.
"Today's surprise ticket release definitely helped the process to be orderly, but some will argue that it wasn't fair."
From 23 May, unsold Olympic tickets of all varieties will be put on general release.
Lord Sebastian Coe told the Andrew Marr show: "Seventy five per cent of the 11 million tickets that available are in the hands of the British public. That's a commitment we made right at the beginning of this process, and at the end of this process we will deliver it.
"There will be disappointment. I accept that. I am a sports fan. There was disappointment, that's why in the opening round of ticket sales we committed to getting as many people who were in those initial ballots across the line.
"There has never been a sports ticket in history that has had such extraordinary demand."
The Games organising committee, Locog, said it would give an update on the ticket situation when the current session finished on 17 May.
It said feedback from those users buying ticket from its website in the latest sale had so far been positive.
BBC News website readers have been discussing their experiences of the sale.
David Beyer from Melton Mowbray said: "I applied for tickets for the men's 100m final night (even though these weren't listed) and was successful in getting three tickets for £295 each.... It will be a fantastic experience for all of us, and hopefully we will see history in the making!"
But James Harvey, from London, said: "No luck again buying tickets from the website: on at 11am on the dot, but all I got was lots of 10-minute waits while it searched for tickets, followed by unavailability.
"All I want is tickets to one event, after all the taxpayer money I've put into this Olympics."
The online ticketing system came in for criticism last year after it crashed under high demand, leaving potential buyers frustrated at long delays.
In some cases, the system reportedly led buyers to believe they had been successful in their application, only to find out by email in June 2011 that they had failed.
From 11:00 BST on Sunday, about one million people who missed out in the first ballot have five days to buy tickets.
According to the original schedule issued by Locog, different tickets were to be available from 11:00 BST each day, as follows:
- 13 May - Hockey, Tennis, Wrestling (Freestyle), Wrestling (Greco-Roman), Beach Volleyball and Football
- 14 May - Athletics (Race Walk), Athletics (Marathon), Canoe Slalom, Canoe Sprint, Shooting, Swimming, Volleyball, Weightlifting and Cycling (Track)
- 15 May - Equestrian (Dressage), Equestrian (Eventing), Equestrian (Jumping), Diving, Synchronised Swimming, Handball, Table Tennis, Swimming (Marathon) and Water Polo
- 16 May - Basketball (North Greenwich Arena sessions), Badminton, Sailing, Gymnastics (Artistic), Gymnastics (Rhythmic), Gymnastics (Trampoline), Judo, Taekwondo, Boxing and Fencing
- 17 May - Archery, Basketball (Basketball Arena sessions), Cycling (BMX), Cycling (Mountain Bike), Cycling (Road), Modern Pentathlon, Rowing, Triathlon and Olympic Park tickets
Tickets that are not sold each day will be carried forward for sale the next day. Once fans are successful they will be unable to buy any more tickets in the five-day period.
Unsold tickets for the Paralympic Games are also set to go on sale from 11:00 BST on 21 May, with further details yet to be announced.
Locog has said it is releasing new tickets for sale from 29 May, to "help more people get to the Games". These tickets are:
- Cycling Road Race at Box Hill, Surrey
- Cycling Individual Time Trial at Hampton Court Palace
- There will also be tickets available for Wimbledon which will provide access to The Hill and the big screen only
- The Orbit - for those who already have access to the Olympic Park on the day they wish to visit the Orbit
The organisers have confirmed there will be a "babes in arms policy" during the Games, which means babies aged 12 months or under will be allowed into all London 2012 venues without a ticket - as long as they are securely strapped to a parent or carer.
But the policy will not apply to events held at Wembley Stadium, St James' Park, Old Trafford and North Greenwich Arena, where licensing rules require everyone to have a ticket.