(ANSA) - Taranto, July 27 - Angry steelworkers in the southern Italian city of Taranto blockaded arterial roads and occupied city hall Friday to protest the partial shutdown of their employer, the ILVA steel plant. Dozens streamed into city hall while others blockaded two main access roads to Taranto, causing traffic jams and disrupting local public transportation, leaving buses stuck or unable to complete their routes. The steel plant's main unions - UILM, FIOM and FIM - called an indefinite strike, leaving just the few workers in command posts to care for key facilities, and called for the intervention of Premier Mario Monti. On Thursday, Taranto judge Patrizia Todisco ordered the shutdown of the mineral park and foundries of Italy's largest steel plant, which press reports say provides roughly 30% of Italy's steel needs and employs over 11,000 people. The same order mandated the house arrest of eight managers and ex-managers, including the owner Emilio Riva. Todisco charged that the steel plant's toxic fumes and dust have endangered the lives of people and animals in the area since 1995. "There is no doubt that those indicted were perfectly aware that steel production activities unleashed harmful, toxic substances (like dioxins) for human and animal health," but "there has been no sign of repentance, since they continued to poison the surrounding environment for years," wrote Todisco in her arrest warrant and plant shut-down order. The prosecution's epidemiological assessment found the steel plant's pollution over 13 years caused 386 deaths, 237 malignant tumors, 247 hospitalizations for heart disease, and 937 hospitalizations for respiratory disease - 638 of which in children. Todisco added in her order that the steel plant also tainted the food chain, polluting nearby goat and sheep pastures. The judge wrote that ILVA's emissions contaminated - with dioxins and PCBs - 2,271 animals destined for food and indirect consumption. Lecce prosecutor Giuseppe Vignola told reporters in Taranto Friday that there was no choice but to shut down the steel plant given the gravity of the epidemiological assessment. Environment Minister Corrado Clini vowed government support to keep the steelworks in production on Friday. "We want production to continue," Clini told Italian news channel Tgcom24. "The cabinet has confirmed its commitment by signing a memorandum of understanding, and the government is working to uphold it," Clini said, making reference to a government pledge Thursday to allocate 336 million euros for ILVA-related environmental clean-up and recovery. The Taranto re-examination court will consider repealing the plant shutdown and arrest orders. The first hearing is scheduled for August 3.