Fiona Hyslop, the Cabinet Secretary for Culture, should "do the decent thing" and overrule any advice to demolish Perth City Hall, the secretary of the Royal Incorporation of Architects in Scotland (RIAS) has said.

Neil Baxter also offered to help find an alternative use for the 100-year-old B-listed building.

Speaking before the body's annual convention in Aberdeen this week, on the subject of "re-engaging" with heritage buildings, Baxter strongly criticised Perth & Kinross Council's plans to raze the hall to make way for the newly-created city's first open plaza. The fate of the 1909 building will ultimately be decided by Hyslop following a recommendation from Historic Scotland expected "in the near future".

Baxter told the Sunday Herald: "My understanding of Scottish historical environment policy is that, before a historic building is demolished, it should have been thoroughly exposed in an unbiased way to the widest possible market for alternative use, and a vigorous marketing process should have occurred. To the best of my knowledge that hasn't happened and, therefore, under present legislation, permission cannot be granted for its demolition. We understand that much political pressure is being brought to bear. We sincerely hope that the Cabinet Secretary will do the decent thing and resist that."

The RIAS proposes that Perth, which has estimated the costs of demolishing the building and replacing it with open space at £3 million, offers a £1m international prize to induce the world's top planners and developers to find an alternative use for the building which, he says "has served Perth very well through the decades".

"The stipulation would be that the building would be for the public benefit of the people of Perth and a requirement that it be sustainable over 25 years, and a prize of £1m could be used by developers to leverage private money.

"There is no legal reason why the local authority can't do that, and the RIAS would be happy to administer the competition."

A spokesman for Perth council said: "Council consultation has shown that a significant proportion of residents and businesses support the creation of a large public open space as being the best option for the future economic health of the city of Perth. We are currently awaiting a decision from Scottish ministers on the plans that councillors have unanimously approved."

Last week, Perth council's former planning director, Denis Munro, warned that Historic Scotland was in danger of "tarnishing their reputation as heritage watchdogs" if it gave the green light to the demolition.

Speakers at the RIAS convention this week include: Morten Schmidt of Danish firm Schmidt Hammer Lassen Architects, builders of Aberdeen University's acclaimed new library building; Japan's Kengo Kuma, architect of the proposed dockside V&A at Dundee; John McAslan CBE, Scots-born architect of the King's Cross station redevelopment; and New York architect Charles Renfro, mastermind of Aberdeen's controversial Union Terrace Gardens renovation project.

Baxter said: "Other organisations have dumped their annual conferences, but we are determined that the convention continues as it has since 1916. There are 3500 practising architects in Scotland, 2500 of whom are RIAS members, and they are at the front end of a sector that makes up between 10% and 15% of Scottish GDP. Thanks to our sponsors, we can invite architects from all over the world and, for a reasonable price, give those attending the opportunity to think about the higher business of architecture."