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Protestant extremists attacked Northern Ireland police and British troops into a third day Monday, littering streets with rubble and burned-out vehicles in violence sparked by anger over a restricted parade.

Crowds of masked men and youths confronted police backed by British troops in dozens of hard-line Protestant districts in Belfast (search) and several other towns. Gunmen shot at police and soldiers in at least two parts of the capital Sunday night, but nobody was hit.

Police units equipped with helmets, body armor and flame-retardant jumpsuits doused crowds with massive water cannons and fired several hundred plastic bullets.

The Police Service of Northern Ireland (search) said 18 officers were injured Sunday night and Monday morning, chiefly by shrapnel from rioters' homemade grenades, raising the force's three-day casualty total to 50 injured.

"This is a moment of choice for everybody. ... Whose side are you on?" said the British governor, Northern Ireland Secretary Peter Hain (search).

"Are you on the side of law and order, applied fairly and equally to every citizen? Or are you against law and order, siding with those firing bullets at the police, throwing petrol bombs and blast bombs at police and attacking them?"

Earlier, Hain received a detailed intelligence briefing on the weekend chaos from Northern Ireland's police commander, Chief Constable Hugh Orde.

Hain said Orde and senior detectives presented "absolutely clear-cut" evidence that members of Northern Ireland's two biggest outlawed Protestant groups, the Ulster Defense Association (search) and the Ulster Volunteer Force (search), were instrumental in directing the riots.

He said he planned to announce within days whether Britain would continue recognizing the legitimacy of the joint UDA-UVF cease-fire, which has existed officially since 1994.

Meanwhile, 16 Protestants arrested during the riots began appearing in Belfast courts Monday to face a wide range of charges, ranging from hijacking vehicles to attempted murder.

Paramedics said they have treated several civilians for gunshot and shrapnel wounds and burns, but only three of them have checked into hospitals — where rioters risk being identified and arrested. One Protestant man shot in the arm Sunday by British troops was arrested on suspicion of attempted murder.

Commuters going into Belfast, home to a third of Northern Ireland's 1.7 million people, listened to radio traffic reports Monday morning that offered a long list of roads and neighborhoods to avoid because they were blocked by burned-out vehicles or littered with tire-puncturing rubble.

Orde has blamed the Orange Order (search) — a legal brotherhood with more than 50,000 members — for inspiring the riots. The violence began Saturday when police prevented Orangemen from parading near a hard-line Catholic part of west Belfast.

But the senior Orangeman in Belfast, County Grandmaster Dawson Bailie (search), said Monday he would not condemn the rioters' actions.

"As far as I am concerned, the people to blame are the secretary of state, the chief constable and the Parades Commission," Bailie said, referring to Hain, Orde and a joint Catholic-Protestant panel that since 1998 has imposed restrictions on Protestant demonstrations.

The American envoy to Northern Ireland, Mitchell Reiss, offered unusually harsh criticism of Protestant political leaders, whom he accused of offering half-hearted, ambiguous backing for the police.

Reiss said Protestant leaders should not be eligible to serve in any future power-sharing government — the central, unfulfilled goal of Northern Ireland's 7-year-old Good Friday accord — unless they stood firmly behind the police and against their voters in the mobs.

"All of us are pretty disappointed with the abdication of responsibility by many (Protestant) unionist leaders," Reiss said in Belfast. "No responsible political leadership deserves to serve in a government unless it supports fully and unconditionally the police, and calls on its supporters to do so."

Police and analysts agreed that the march provided a pretext for the UDA and UVF to launch a pre-planned rebellion against police authority. Their desire for street mayhem reflected their near-total disconnection from the British province's decade-old peace process.

The UDA and UVF are both supposed to be observing cease-fires and disarming in support of the 1998 peace pact, just like the outlawed Irish Republican Army (search) rooted in militant Roman Catholic areas.

But while the IRA has built a major base of support through its Sinn Fein (search) party and has grown central to ongoing negotiations on Northern Ireland's future, the Protestant paramilitary groups have dismally failed to win electoral support and barely register in political talks. Instead, they wield power through criminal graft backed by occasional intimidating shows of force.