A hardcore of Lazio and AS Roma soccer fans worked together to spark the riot that caused the Rome derby to be abandoned on Sunday, politicians said on Monday.

Police on Monday said they had arrested 13 supporters from both sides, some of them known hooligans, following a six-hour pitched battle between police and fans that left more than 170 people injured.

Some politicians connected the violence to Prime Minister Silvio Berlusconi's comment a day earlier that "revolution would explode" on the terraces if the government did not help out Italian soccer that is 510 million euros ($629.5 million) in debt.

"It seems to me like an operation aimed at blackmailing the government. It was planned to place strong pressure at a moment when the executive must make an important decision," Welfare Minister Roberto Maroni said.

Violent scenes erupted when the game was abandoned in the second half after a false rumour circulated that a young boy had been killed by a police car outside the Olympic Stadium.

In an unprecedented move, AS Roma captain Francesco Totti came off the pitch to speak to a fan who told him about the false story and he then asked for the match to be stopped.

Police denied the rumour over the public address system but some Roma fans set fire to the stands, sending terrified supporters streaming out over the terraces.

Outside the stadium hundreds of fans wearing Roma and Lazio colours hurled flares, bottles and knives from behind makeshift barricades. Police responded with volleys of tear gas.

GUERRILLA WARFARE

"It was true guerrilla warfare," one police source said.

A total of 155 police officers were wounded and 21 fans were hurt, police said on Monday.

Police searching the grounds outside the stadium on Monday said they had found paper bombs stuffed with nails, screws and metal shards as well as knives, bars and sticks.

They added that the number of arrests could rise as officers looked at close-circuit television footage.

Italian Football League president, Adriano Galliani ordered the match to stop via telephone from Milan for reasons of public order.

But stadium security staff had said there was no risk in continuing the game.

"I suspect that all that happened was pre-arranged: in politics you call it a strategy of tension," Culture Ministry Undersecretary Mario Pescante said.

Some critics blame Berlusconi for the financial problems in Italian soccer by setting the example of huge salaries at AC Milan that has led to some clubs being unable to pay taxes.

The European Union had already moved to limit previous government efforts to help out Italian soccer which also faces a series of fraud investigations.