Outside the former US embassy’s compound in Tehran – now a museum called “den of spies” – hundreds of Iranians gathered last week to celebrate the 35th anniversary of the hostage crisis, when 52 American diplomats were held captive for 444 days.

A number of senior Iranian officials, mostly from the Conservative Front, were among the crowds railing against “the Great Satan”. But President Hassan Rouhani, whose government has engaged in direct diplomacy with the US over Tehran’s nuclear programme, was not among them.

There remains a deep suspicion of the US in Iran, but even the most conservative factions of the Islamic republic hope for a way out of the current nuclear stalemate. A few hours of marching and chanting “death to America” and “death to Israel” ended with the participants issuing a statement expressing their support for the president’s nuclear diplomacy and his negotiating team, who are travelling to Vienna next week in what amounts to a critical moment in the latest round of nuclear talks. Speculation is high about what will happen there.

Mohammad Marandi, a Tehran University professor close to the Iranian establishment, told the Guardian that Iranians wanted the negotiations to succeed and that in his view, the Americans could not afford for them to fail. “I don’t think that the talks will fail because the United States is facing too much trouble. Americans and Europeans are facing major economic problems at home, tensions with Russia are very high, Russia and China are moving towards Iran, tensions with China are gradually increasing over the South China Sea and of course the US-Saudi alliance has led to the Isil [Isis] disaster that is a major security threat to western countries. I don’t think the Americans are really in a position to escalate tensions.”

Seyed Hossein Mousavian, a former spokesperson for Iran’s nuclear negotiating team who has worked with Rouhani in the past, said a nuclear agreement in Vienna would strengthen the Iranian president’s administration. Rouhani owes his victory in last year’s presidential elections to promises of ending the nuclear crisis.

Mousavian told the Guardian: “It would have a great impact on the economic situation, improve the image of the US and the west in Iran, would open the door for broader cooperation with the world powers on the regional crisis and would be a great achievement for the Iranian administration.”

Last month, a group of Iranian and foreign investors gathered in London’s Iran-EU forum studying business opportunities in the Islamic republic should the talks bear fruit and international companies could return to Iran. Esfandyar Batmanghelidj, who organised that event, struck a more optimistic note and said investors were hoping for a return.

“I think the mood is cautiously optimistic,” he told the Guardian. “Everyone is following the talks. There’s an interesting situation where they are following the political analysis, they are keeping up to date with the news and people are hopeful the P5+1 [UK, China, France, Germany, Russia and US] and Iran are serious about coming to an agreement but however many positive signs there are, there are also worrying signs like the outcome of the elections in the US and the very significant resistance to the deal that is coming from certain special interest groups.”

According to Batmanghelidj, if the negotiations lead to a comprehensive deal that includes easing sanctions, there will be a great deal of business opportunities in Iran.

“Typically when individuals are talking about the opportunities in Iran, the first thing people think is of is big oil and energy contracts but I think the real opportunities and the one that is going to mean the most for Iran in the long term in terms of it developing into an advanced economy, is opportunities around Iran’s consumer marketplace,” he said.

“It’s a country of 80 million people – young, educated, they have pretty good purchasing power. They are hungry for the best international products and brands, everything from cosmetics to cars to healthcare products, I think the industries that touch the end consumer are going to be the ones that are most successful.”

Amir Ali Handjani, an Iranian-American energy executive based in Dubai, said foreign companies were waiting on the sidelines to see if the negotiations with Iran and the P5+1 culminate with a deal.

“Until then multinational companies will remain on the sidelines and look for a clear and unambiguous signal from their respective governments that the political process is going forward in a positive direction and it’s permissible to enter the Iranian market – without that green light the risk far outweighs the rewards,” he said.

“I think if there’s a deal, internally there’s going to be some jockeying for position to both implement it and block it by different factions in Tehran. I do think that if there’s a gradual lifting of sanctions, business will return to Iran as long as they receive a green light from their respective governments that they will not be penalised, fined or targeted for doing business with Iran.”

But if the talks fail, Handjani said Iran’s economy would suffer a lot. “If the talks fail, Iran is going to suffer tremendously because the economy is hitting a perfect storm of sanctions, lower oil prices, inflation and devaluation of the rial. With the declining price of oil, Iran’s revenue base is dwindling. It is not able to sell its oil unencumbered and when it does it is not able to access the proceeds of its sale freely.”