A rally at Belfast's Waterfront Hall yesterday drew more than 2,500 people in support of the Ashers Baking Company, whose legal case begins tomorrow in the High Court.

An action is being brought against the Christian-owned bakery by the Equality Commission because it refused to bake a cake with a slogan in favour of gay marriage on it.

Speakers at the event included bed and breakfast owner Hazelmary Bull, who was successfully sued when she and her husband refused to let a gay couple share a bed.

The issue has polarised political opinion in Northern Ireland, with First Minister Peter Robinson criticising the Commission for bringing the case. "The issue here is where there are competing rights, ensuring that there is reasonable accommodation. That is what the Equality Commission have missed in all of this," he said.

"When you consider that they have set aside the potential of spending £33,000 on this court case where they are seeking damages of £500 against Ashers, there is a better use that could be put to that money, particularly in the tight fiscal situation the Executive faces," Robinson added.

The Commission told the Belfast Telegraph that it had so far spent £8,586.60 on the case, adding that it had "an important role in ensuring effective application of Northern Ireland's equality laws and supports cases so that people are aware of, and can avail of, the protection these laws afford against all forms of unlawful discrimination".

However, a religious conscience clause bill tabled by DUP MLA Paul Givan is likely to be blocked by Sinn Fein and the SDLP on the grounds that it would entrench discrimination, a charge denied by supporters. Green Party MLA Steven Agnew said: "The so-called 'conscience clause' is nothing but a thinly veiled attempt to legalise discrimination against LGB people."

The case has also divided legal opinion, with Aidan O'Neill QC saying that the legal action could have far-reaching implications for "negative freedom of expression" because it could see could see a Muslim printer ordered to print images of the Prophet Mohammed or an atheist web designer forced to create a religious website.

However, in a Belfast Telegraph opinion piece, human rights lawyer Prof Steven Greer wrote that there was "little doubt" that Ashers would lose its case. He added: "Were Mr Givan's amendment to be passed, same-sex couples in Northern Ireland could, for example, be lawfully denied a table at a restaurant, a room in a hotel, or a mortgage, on the grounds that this would otherwise endorse or facilitate same-sex unions." A conscience clause, he said would be open to repeated legal challenges.

He suggests: "Those providing goods and services in Northern Ireland who do not approve of gay relationships, could and should simply issue a disclaimer on their invoices, websites, and other business documents to the effect that compliance with relevant statutory requirements does not necessarily constitute approval of the activities in question."