By John Irish and Clotaire Achi

PARIS (Reuters) - Thousands of opponents to Iran's authorities rallied in Paris on Saturday, joined by Ukrainians to call for the fall of the government in Tehran, hopeful that U.S. President Donald Trump's 'maximum pressure' campaign could lead to change in the country.

The protest, organised by the Paris-based National Council of Resistance of Iran (NCRI), which is banned in Iran, comes as two of the group's members face imminent execution with a further six sentenced to death in November.

"We say your demise has arrived. With or without negotiations, with or without nuclear weapons, uprising and overthrow await you," NCRI President-elect Maryam Rajavi said in a speech.

People from across Europe, often bussed in for the event, waved Iranian flags and chanted anti-government slogans amid images deriding Iran's Supreme Leader Ali Khamenei.

Hundreds of Ukrainians accusing Iran of backing Russian President Vladimir Putin in the war against Ukraine joined the protest.

Iryna Serdiuk, 37, a nurse turned interpreter originally from the embattled Donbass region, and now exiled in Germany, said she had come to Paris to join forces against a common enemy.

"I'm happy to see these Iranians because they are opposition. They support Ukraine and not the Iranian government which gives Russia weapons. We are together and one day it will be victory for Ukraine and Iran too," she said.

The NCRI, also known by its Persian name Mujahideen-e-Khalq, was once listed as a terrorist organisation by the United States and the European Union until 2012.

SUPPORT INSIDE IRAN

While its critics question its support inside Iran and how it operates, it remains one of the few opposition groups able to rally supporters.

Mohammad Sabetraftar, 63, an Iranian who has been in exile for 40 years and now runs a taxi business in the United Kingdom, dismissed criticism of the NCRI saying that it was the only alternative capable of achieving democracy in Iran.

"What we expect from Mr Trump or any Western politician is to not support this government. We don't need money, we don't need weapons, we rely on the people. No ties with the regime, no connections and put as much pressure on this government."

Tehran has long called for a crackdown on the NCRI in Paris, Riyadh, and Washington. The group is regularly criticised in state media.

In January, Trump's Ukraine envoy spoke at a conference organised by the group in Paris.

At the time he outlined the president's plan to return to a policy of maximum pressure on Iran that sought to wreck its economy, forcing the country to negotiate a deal on its nuclear and ballistic missile programmes, and regional activities.

Homa Sabetraftar, 16, a schoolgirl in Britain, said she felt it was her duty to come to the event to represent the youth of Iran.

"Some people in Iran don't have that voice and aren't able to vocalise as freely as we are able to here," she said. "We need to push for a better future."

(Reporting by John Irish; Editing by Emelia Sithole-Matarise)