By John Irish

PARIS (Reuters) -The last heir to the Iranian monarchy urged Western states on Monday to accept that the collapse of the current Iranian authorities is necessary to deliver lasting peace and regional stability.

The United States, which bombed Iranian nuclear sites on Saturday, has said it wants to prevent Iran developing nuclear weapons, not to open a wider war.

Officials in Washington said the goal of the U.S. bombing was not "regime change" but, in a social media post on Sunday, U.S. President Donald Trump raised the possibility of Iran's hardline clerical rulers being toppled.

"Now is the moment to stand with the Iranian people. Do not repeat the mistakes of the past. Do not throw this regime a lifeline. The destruction of the regime's nuclear facilities alone will not deliver peace," Reza Pahlavi, the exiled son of the toppled Shah, told a press conference in Paris.

"You are right to be concerned about stopping nuclear weapons and securing regional stability, but only a democratic transition in Iran can ensure these goals are achieved and are lasting."

The Iranian authorities did not immediately comment on Pahlavi's remarks.

Pahlavi has lived in exile for nearly four decades, since his father, the U.S.-backed shah, was overthrown in the 1979 Islamic Revolution.

It is unclear how much support Pahlavi has in Iran. Many Iranians remember the Shah's repressive secret police, Savak, and there have been pro- and anti-monarchy slogans during mass demonstrations in Iran in the past.

Without providing evidence, Pahlavi, who is based in Washington, said the ruling system in Iran was collapsing and that Supreme Leader Ali Khamenei, his family and other senior officials were making preparations to flee the country.

"This is our Berlin Wall moment. But like all moments of great change, it comes fraught with danger," he said, referring to the collapse of the wall that divided East and West Berlin in 1989 as the Soviet-led Communist bloc crumbled.

FRAGMENTED OPPOSITION

Iran and Israel have been waging an air war since Israel launched airstrikes on June 13, saying it wanted to prevent Iran developing nuclear weapons. Iran says its nuclear programme is solely for peaceful purposes.

Asked whether he wanted to lead a future transition or be a new Shah, Pahlavi said he was not seeking political power.

He said he saw the cornerstone of a transition based on Iran's territorial integrity, individual freedoms and equality of all citizens, as well as separation of religion and state.

Opposition to Iran's clerical government is fragmented, with no clear recognised leader and a multitude of ethnic groups.

Pahlavi said his teams were working on a future economic plan and that he wanted to convene a national unity gathering that would include activists, dissidents and groups from across the ideological spectrum to agree on the transition principles.

It would also bring together business leaders, professionals and experts, he said, without giving a time frame.

He said he had also created a platform for Iranian security, police and military officials to join him if they wanted to abandon the government.

Asked about hesitations among some Western leaders over the potential consequences of the authorities collapsing, he said: "It's not that they need to advocate for regime change. It is that they simply have to recognise that regime change is the only ultimate solution."

(Editing by Timothy Heritage)