Twenty thousand students marched through London on Wednesday because they do not want to live in a society where access to education is based on wealth, said National Union of Students president Mandy Telford.
She said: "Top-up fees will quite simply cripple our higher education system. With fees at this level, it is clear that many people will not be able to afford university in the future."
Students were supported at the rally by Fire Brigades Union chief Andy Gilchrist.
Former health secretary Frank Dobson said it was imperative to avoid the emergence of "cheque-book entry" to the best universities. Interviewed on BBC Radio 4's Today programme, he warned that top-up fees would "put off lots of potential students, particularly ones from badly off homes, but also ones on middling incomes".
At an Aberdeen University fundraising event, the Scottish secretary, Helen Liddell, warned against students being treated as cash cows.
Will Straw, Oxford student union president and son of foreign secretary Jack Straw, said at the march: "What we must see is the government putting its money where its mouth is. If it wants to expand to 50 per cent participation in higher education, it has to pay for it."