For the first time, many leading institutions will be competing for bright students after government reforms meant no limits on the number of students with at least ABB grades.
The UK's top universities, which include many Russell Group institutions, were taking part in the clearing process to snap up able students who were still looking for places, or seeking to trade their offer.
Almost 30,000 courses were being advertised on the clearing website yesterday afternoon and 145,730 applicants were eligible for the process, which matches students without places to degrees with vacancies.
Students could begin looking at courses available in clearing and contacting universities about vacancies yesterday, but could not begin making choices through the Ucas website until 5pm last night, with more than 6,000 choices added in the first 10 minutes.
Ucas chief executive Mary Curnock Cook said a lot of the advertised courses would "only be for people who have the very highest grades".
This is because government reforms mean there is no limit on the numbers of students with at least one A and two B grades at A-level that universities can recruit.
But she added: "The reality is that there are vacancies across all sorts of courses and institutions."
The major overhaul of higher education means that a number of top institutions that do not always enter clearing are taking part in the process to compete for top students.
A snapshot survey of the Ucas clearing website taken yesterday afternoon by the Press Association found that more than one in 10 courses listed with vacancies were for Russell Group universities.
The Russell Group represents 24 leading institutions including Oxford and Cambridge, neither of which are taking part in clearing.
In total, 16 of the 24 universities were advertising vacancies, with more than 3,400 courses on offer for students in England between them.
Russell Group director general Dr Wendy Piatt said: "We hope this year's change to a lower threshold of ABB or equivalent will reduce some of the unintended consequences from last year when students who wanted to attend a leading university and had the right qualifications were not able to - even when those universities wanted to accept them.
"One consequence of the uncertainties in the new system is that universities may have more places to offer through clearing to well-qualified students who have narrowly missed out on their first choice.
"Ucas and our universities have been preparing for this carefully and are on hand to help students who have missed their offer.
"There may also be places available through the Ucas adjustment process for those who have done better than expected.
"We will be monitoring this carefully over the coming days and weeks."
The national A-level results for England, Wales and Northern Ireland, which were published yesterday, showed that the proportion of A-level grades scoring an A or higher had fallen for the second year running as rising numbers of students opted for traditional subjects like science and maths.
In total, 26.3% of entries scored an A or A*, down from 26.6% last year - a drop of 0.3%.
It is believed to be the second biggest fall in the history of A-levels, and comes the year after the A*-A pass rate fell for the first time in more than 20 years.
The number of entries awarded an A* - the highest grade - also dipped to 7.6%, compared with 7.9% last year, while the overall A*-E pass rate rose slightly by 0.1% to 98.1%.
A breakdown by subject revealed a continued move towards science and maths A-levels, which are often seen as tougher, and more traditional subjects.
There were almost 24,000 more entries for the sciences this year compared with 2009, the Joint Council for Qualifications (JCQ) said, and nearly 19,000 more for maths courses.
It also emerged that boys outperformed girls in the top grade again, and widened the gender gap, with 7.9% of boys' entries attaining an A* compared with 7.4% of girls.
Young people continued to turn their backs on many modern foreign languages, with French and German entries down again, although Spanish bucked the trend with numbers up 4.1%.
Exam boards announced an investigation into the issue.
As of first thing yesterday morning, 385,910 applicants had been accepted by UK universities and colleges - 31,600 more than at the same point last year, a rise of 9%.
Ucas said it was the highest number of acceptances recorded on A-level results day.