Britain, preparing for a feared upsurge of violence in Northern Ireland and possibly elsewhere following the funeral Thursday of Irish nationalist terrorist Bobby Sands, was airlifting 600 crack troops tonight to help back up Ulster police.

Terrorist snipers killed a policeman and wounded a policewoman and a samll child in Belfast tonight and an explosion in the city's market district killed a young men who police said was probably carrying a bomb. A dozen residents were treated for shock and cuts from windows shattered by the blast. The snipers also wounded two soldiers in the town of Crossmaglen near the border with the Republic of Ireland as sporadic rioting and the hijacking and burning of vehicles continued.

The arrival overnight of the 1st battalion of the Royal Regiment of Wales marks a reversal in what had been a steady reduction in the number of British soldiers here from a peak of nearly 22,000 at the height of sectarian violence in 1972. The battalion will bring British military strength in the province to nearly 11,700 troops.

Security also was tightened for government offices and public figures in London and elsewhere in Britain, according to officials, to protect against a "spectacular" terrorist act by the Provisional Irish Republican Army. IRA member Sands went on hunger strike demanding political prisoner status for himself and hundreds of other convicted and terrorists imprisoned here. He died yesterday after 66 days without food.

Three others in the Maze Prison near Belfast remain in week condition on hunger strikes that have lasted 46 to 52 days.

In a preview of Thursday's funeral and a three-mile procession through mostly Catholic neighborhoods in west Belfast to a cemetery where other IRA members are buried, Sands' body was moved tonight from his parents' house to the nearby Catholic church of St. Luke's in a housing project on the western edge of Belfast.

A bagpipe played and six men in paramilitary uniforms with masks and black berets followed as the coffin was carried several hundred yards in a cold drizzle through a crowd of several hundred relatives, friends, neighbors and sympathizers.

On the tricolor flag of the Irish Republic covering Sands' coffin were a Bible, and a black beret and black gloves symbolizing his commitment to the terrorist organization that seeks to force Britain out of Northern Ireland and a union with the republic to the south.

Among the spectators who then filled the church to overflowing for a short service were many families with small children who had walked from other nearby Catholic housing projects.

Across a vast open field serving as a dividing no-man's-land, Protestants in another housing project stood sullenly outside their homes and watched the distant procession. A winding road linking the two projects was barricaded by overturned, burned-out cars.