* ILVA appeals court seizure of steel
* Says faces "immediate" shutdown of plant
* Prime Minister to meet with factory head on Thursday
* Police arrest seven, including top managers (Adds government meeting)
By Vincenzo Damiano
BARI, Italy, Nov 26 (Reuters) - Italy's ILVA said it may have to close Europe's biggest steel plant after judges ordered the seizure of steel and semi-finished products in a corruption probe that saw several managers arrested on Monday.
Prime Minister Mario Monti will meet with the plant's president on Thursday in response, reflecting the sensitivity of the case for the technocrat government. At stake are some 20,000 jobs in an area already blighted by high unemployment and a stagnant economy.
The meeting will bring together the environment, labor and health ministers, regional governors and union chiefs, the government said in a statement.
Prosecutors ordered the arrest of seven people on suspicion of bribing officials to cover up a health and environmental scandal at the sprawling site in Taranto in southern Italy, where emissions of cancer-causing chemicals have been blamed for abnormally high cancer rates and respiratory diseases.
Justice system officials said the prosecutors arrested Emilio Riva, chairman of the group that owns ILVA, and six others including his son Fabio.
President of the factory Bruno Ferrante, who is due to meet Monti on Thursday, was also placed under investigation over the scandal.
"I have no intention of abandoning my position as president of ILVA. The accusations made against me by the Taranto prosecutors are without substance," he said on Monday.
No comment was immediately available from the Riva group.
Officers from the Guardia di Finanza also impounded finished and semi-finished products intended for sale or transfer to other parts of the group, which also operates production sites around Genoa in northwestern Italy.
The order covers steel produced at the plant since its furnaces were placed under special administration four months ago following accusations it was responsible for serious health problems in the region.
"I am not surprised this could force them to close the plant. Four months worth of cash flow would be lethal for any steel company right now," said one industry analyst, who declined to be named.
The group said it would appeal against the decision, which made it impossible to sell its products and would lead to the "immediate and unavoidable" closure of the Taranto factory and all other parts of the group which depended on it.
The powerful business lobby Confindustria said a closure would be an "extremely grave event for Italian industry resulting from true judicial harassment".
The arrests were the latest twist in a saga that has threatened the future of ILVA, a vast industrial site that employs 12,000 people directly and keeps another 8,000 in work in one of the largest industrial sites in southern Italy.
ILVA has been seen as a test case of the government's ability to protect Italy's heavy industry and preserve one of a shrinking number of major manufacturing employers in the poor and underdeveloped south.
Prosecutors suspect ILVA executives paid bribes to local politicians, officials and business people to hide the scale of the environmental disaster highlighted by a series of damning environmental reports.
Emissions from the plant have been blamed for abnormally high levels of tumors and respiratory diseases in the region, prompting protests from local people faced with a seeming choice between jobs and health.
The government last month gave ILVA authorization to continue operations on condition that it cut emissions and cleaned up the plant.
Metalworkers' unions called on Monti to act to save jobs and said the government had to "declare clearly whether it wants to save an industrial and employment centre which is essential to the country."
ILVA said on Monday the plant fully conformed with safety regulations and denied its operations were in any way connected with any abnormal mortality levels. (Additional reporting by Silvia Antonioli and Naomi O'Leary; Writing by James Mackenzie; Editing by Dan Lalor, Helen Massy-Beresford and Tim Dobbyn)