* Top Iran shipping line hit by rial's plunge
* Growing criticism of Ahmadinejad's handling of economy
* Seaborne trade crashes more than 50 percent
Iran's vital seaborne trade is buckling under the weight of Western sanctions, deepening hardship for a population deprived of basic imports and heaping intense pressure on Tehran over its nuclear programme.
Many of Iran's imports, including food and consumer goods, arrive on container, bulker and other ships, but the number of vessels calling at its ports has dived by more than half this year as the United States and European Union tighten the screws.
Analysts doubt the Iranian economy is near collapse, even though its rial currency has plunged in the last few weeks, but they say some shortages and rising prices of imported goods could provoke public unrest directed at Tehran's leadership.
A growing number of Western companies, especially those in shipping and related businesses, are pulling out of trade with Iran due to the complexities of deals and tougher banking restrictions as the sanctions take hold - and out of fear of losing business elsewhere.
"Iran's commercial shipping sector has suffered a significant hit," said Anthony Skinner of risk analysts Maplecroft.
"Although U.S. and EU sanctions do not target food shipments, importers struggle to acquire letters of credit and transfer funds. I expect current sanctions and the further tightening EU sanctions to sour the appetite of the international commercial shipping sector further."
The United States and the EU have led the sanctions push, hoping to force Iran to halt its nuclear programme which they suspect is aimed at making weapons. Tehran says the work is peaceful, but the trade measures are hurting shipping badly.
Data from maritime intelligence publisher IHS Fairplay showed the overall number of vessels calling at Iranian ports in the year to early October was 980. That figure for more than three quarters of this year compares with 2,740 ships for the whole of 2011 and 3,407 for 2010.
Of that total, the number of visits by container ships - which carry consumer goods ranging from foodstuffs and household items to clothing and toys - was 86 so far this year, compared with 273 for the whole of 2011 and 378 in 2010.
The world's top container firm Maersk Line said this week it had stopped port calls to Iran, citing the risk of damaging trade opportunities especially in the United States.
"Lower shipping volumes may also mean that importing vital commodities will be increasingly hard, leading to possible riots over inflation," said Alan Fraser, Middle East analyst with security firm AKE.
Only eight refrigerated cargo vessels carrying fresh produce including bananas called at Iranian ports so far this year, down from 16 in 2011 and 36 in 2010, the IHS Fairplay data showed. Even fishing trawlers unloading their catch have slumped to five from 14 last year and 20 in 2010.
Starved of dollars as the sanctions curb oil exports, Iran bought large amounts of grain earlier this year using other currencies. Nevertheless dry bulk ships, which can carry cereals and commodities such as coal and iron ore, have also made fewer port calls with 100 arrivals so far compared with 352 in 2011 and 406 in 2010.
"You start to see Iran reaching a balance of payment crisis particularly on the imports side when a plummeting currency, which makes imports exceedingly expensive, is compounded by external sanctions," said Mark Dubowitz with the Washington-based Foundation for the Defense of Democracies.
"The combination of these factors is making it difficult for Iran to buy what it needs from abroad and pay for these goods and services," said Dubowitz, who has advised U.S. President Barack Obama's administration and U.S. lawmakers on sanctions.
"ALMOST A BLACK HOLE"
Official Iranian data is not readily available and officials in the country could not be reached for comment. Iran's fleet has taken steps to camouflage its sea trade to discourage foreign attention.
"Iran is almost a black hole these days and it's hard to find even reliable schedules for their main container liner," a European ship industry official said. "They are doing everything they can to remain invisible, including changing the names of their ships to discourage people from tracking them."
In just 10 days up to last week, the rial plunged about 35 percent in the free market to a record low against the dollar, a consequence of Iran's declining oil income.
Mohammad Hussein Dajmar, managing director of the Islamic Republic of Iran Shipping Lines (IRISL), said this week that the central bank had blocked $50 million of the company's assets - reflecting the acute shortage of U.S. currency.
This is restricting the supply of foreign currency, which Iran's top commercial cargo shipper needs to pay bills outside the country and keep operating.
"Unfortunately this step by the Central Bank presents many problems for the company," he was quoted as saying by the Iranian Students' News Agency. "The Central Bank is holding this amount as foreign currency and is only ready to return the rial equivalent of it to us," he said.
On top of this, the central bank would pay the rials at the official rate of 12,260 to the dollar, far from the open market rate of more than 30,000.
IRISL, which has been on a Western blacklist of sanctioned entities for a number of years, denies any wrongdoing and has struggled with the tougher conditions.
Supreme Leader Ayatollah Ali Khamenei told Iranian officials this week to stop bickering over the economic problems. These have fed criticism of President Mahmoud Ahmadinejad by political enemies eager to pin the blame on his administration.
"Sanctions are a long way from causing economic collapse," AKE's Fraser said. "However, its (Tehran's) options are narrowing and internal criticism is likely to produce growing splits within the different political camps. The Ahmadinejad camp looks like it could be the most likely casualty of these."
Iran can also bring in goods overland but business intelligence firm Business Monitor International (BMI) has forecast imports still falling 10 percent this year.
It also estimates a 5 percent drop in 2013 for a country with a growing population, compared with a 1.2 percent rise estimated for last year. BMI forecast Iran's economy will contract 1.2 percent and 0.3 percent in 2012 and 2013 respectively versus estimated growth in real terms of 0.7 percent in 2011.
"As a result of EU and U.S. sanctions on the oil sector, export growth will fall dramatically and this will be felt at the country's ports," said BMI shipping analyst Daniel Richards.
"In addition, tighter sanctions on the financial industry will deter investment and private consumption will stay subdued, due in large part to high inflation, which will curb imports of containerised goods into the country."
MORE PAIN AHEAD
Iran has faced an exodus of international companies providing marine-related services including certification of its fleet, which is vital for securing insurance and ports access. Earlier this year, sanctions pressure also led to the near collapse of an Iranian-led shipping venture with an Indian firm.
Another package of proposed sanctions by the United States and EU aims to tighten the economic noose on Tehran yet further.
"There is strong momentum now to target non-humanitarian commercial imports to Iran in order to put significant pressure on its balance of payments and bring the economic cripple date closer," Dubowitz said.