* Non-Aligned Movement summit opens in Tehran
* Iran assumes 3-year presidency, hopes for solidarity
* Syria likely to dominate agenda, divide participants
Iran welcomed a group of 120 developing nations on Sunday to a summit it says proves that Washington has failed to isolate it from the rest of the world.
Opening the Non-Aligned Movement (NAM) meeting in Tehran, Foreign Minister Ali Akbar Salehi said he hoped for a show of solidarity against sanctions the West has imposed to punish Iran for its nuclear activities.
"The non-aligned (movement) must seriously oppose ... unilateral economic sanctions which have been enacted by certain countries against non-aligned countries," Salehi told the summit's opening session.
Western diplomats have sought to downplay the importance of the summit and the start of Iran's three-year presidency of NAM, a body set up in 1961 to counter big power domination of international relations.
But 80 countries are participating in the summit at the level of minister or higher, Foreign Ministry spokesman Ramin Mehmanparast said, with 50 sending their heads of government.
And the expected attendance of big players including U.N. Secretary General Ban Ki-moon and Egypt's new president, Mohammad Mursi - the first Egyptian leader to visit Iran since the 1979 Islamic revolution - will give the meeting diplomatic heft.
Since the toppling of Egypt's Western-backed President Hosni Mubarak last year, non-Arab Iran has hoped for a thaw in relations with the regional power, but Cairo has appeared less eager to embrace Shi'ite Muslim Iran which is viewed with suspicion by its Sunni Gulf Arab neighbours.
"The presence of the Egyptian president in Tehran will help develop Tehran-Cairo relations," Mehmanparast told reporters.
Divisions within the Muslim Middle East were also evident when Tehran moved to deny media reports that it had invited Ismail Haniyeh, the leader of Hamas, the Islamist group that controls the Gaza Strip.
"From Palestine, only Mahmoud Abbas, the president of the Palestinian Authority, has been invited to Tehran for participation in this summit, and an official invitation has not been sent to any other individual," Mehr quoted Mehmanparast as saying.
With the Syrian crisis likely to dominate talks, Iran's support for President Bashar al-Assad, who is using heavy weaponry against an uprising that threatens to have repercussions around the region, is likely to face scrutiny.
Mehmanparast said Iran expected to consult with countries on the sidelines of the summit on a "comprehensive package" to resolve the Syria crisis.
Tehran has declared a five-day holiday in the capital to ease traffic pressures and potential security problems.
Conservative cleric Ahmad Khatami warned opposition activists, in an interview published last week, not to use the NAM summit as a chance to rekindle their protests which brought huge numbers to the streets in 2009 to protest against the re-election of hardline President Mahmoud Ahmadinejad.
An opposition group based outside the country has asked U.N. chief Ban to ask to meet opposition leaders Mirhossein Mousavi and Mehdi Karroubi who have been held under house arrest since February 2011 as the Arab Spring uprisings raged in the Middle East.