Hundreds of thousands of Protestants led by the Rev. Ian Paisley brought British-ruled Northern Ireland almost to a halt today in a wave of demonstrations against British failure to crush Irish Republican Army violence and government moves they fear will unite Ulster with the Irish Republic.

"We are not going into an Irish republic, never, never, never," Paisley told a crowd of 10,000 at a midday meeting outside the city hall of Belfast, the provincial capital. "It will be over our dead bodies."

Paisley launched his "third force" of vigilantes, some of whom are armed, to defend the province's Protestantsagainst IRA gunmen who have been conducting a systematic campaign of terror in recent weeks.

He said the British government had done nothing to protect the Protestants, especially Robert Bradford, a member of the British Parliament who was gunned down by the IRA last week.

Protestants make up two-thirds of Ulster's population.

The Protestants are up in arms over a recent meeting in London between British Prime Minister Margaret Thatcher and her Irish counterpart Garret FitzGerald, at which they agreed to set up an Anglo-Irish governmental council to discuss "matters of common concern."

They also said they would set up a committee of private citizens and hoped an interparliamentary committee would also be established, with the clear aim of trying to reconcile Britain and Ireland after centuries of violence and bitterness.

Thatcher insisted that no move would be taken to change the status of Northern Ireland as a province of Britain without a referendum. But the Northern Irish Protestants saw the decision as a start on the slide toward the absorption of Northern Ireland into the predominantly Roman Catholic Irish Republic.

The slogan of the "third force," which held its first parade of about 4,000 masked men in paramilitary dress in the town of Newtownards during Paisley's "day of action," is "For God and Ulster."

They marched briskly, three abreast, in platoons of about 100 men and swiftly obeyed barked orders indicating that they had undergone some training.

Meanwhile, the IRA showed its ability to continue its campaign of terror in London this morning. In an ugly new twist to its traditional bomb attacks, the IRA left an explosive device inside a toy gun on the street outside a barracks in suburban Woolwich.

It was set off when a dog sniffed it, and two soldiers' wives nearby were injured, one of them seriously.

In Northern Ireland, work stoppages were widespread. All 400 manual workers at the province's bigger power station walked off their jobs, but the electricity supply was maintained. Many factories emptied. Offices closed, ferries were canceled and schoolchildren in several areas were sent home early.

Major towns, most of them Protestant-dominated, were brought to a standstill by car and tractor cavalcades that choked the streets and processions led by pipe and drum bands.

Paisley dominated the day's proceedings, which were seen by many as a move on his part to bolster his bid for leadership of Northern Irish Protestants.

But many more moderate Protestants held their own rallies and criticized him. Harold McCusker, who like Paisley is a member of the British Parliament, denounced him for shouting from the House of Commons gallery and interrupting speeches last week. Paisley and two of his allies were suspended from Parliament for five days after their outburst.

McCusker said the policies of the British government would not be changed by "insulting it and abusing the privileges of our own Parliament, and those who do so are doing us more harm than our enemies."

Another anti-Paisley leader, James Molyneaux of the official Unionist Party, read out a declaration sent to the queen denouncing Britain's failure to stop the IRA killings and begging "your gracious majesty to instruct your ministers to abandon any Anglo-Irish institution."

He also called for the restoration of the death penalty for terrorist murders.

In Dublin, Prime Minister FitzGerald commented that "Dr. Paisley has his own particular tactics designed to further his own political aims."

"He and the IRA work in tandem with each other," he added. "They are the influences bringing Northern Ireland down."