A bobby-trapped van exploded in the predominantly Roman Catholic working class area of the city today killing one police officer and severely injuring three others. Political tension rose as jailed terrorist Bobby Sands' condition worsened in the 58th day of his hunger strike.

The Irish National Liberation Army, an extreme leftist offshoot of the outlawed Irish Republican Army, took responsibility for the bombing.

Police swooped down on various pro-IRA activists in Northern Ireland today, arresting up to 20 people who have been prominent in the campaign to marshal public support for protesting IRA prisoners. Other activists reportedly have gone underground following the raid.

In another development, the British Army reported finding an IRA arms cache in the Catholic neighborhood of Armagh with grenades, pistols, rifles, bomb-making equipment and ammunition. Two men were arrested.

As Sands nears death, as a result of a campaing to win political status for IRA inmates at the Maze prison near Belfast, British officials are facing what many observers say could be the worse violence to hit Ulster in the last 12 years.

The IRA's political wing accused the British government of planning to let Sands starve to death rather than come to a settlement of his demands.

All police leaves have been cancelled in Ulster, and although a British official said today that no extra troops have been brought in, a special battalion is ready in England if needed.

Meanwhile in London, police warned of a possible campaign against political figures after Conservative member of Parliament Barry Porter received a letter bomb in the mail yesterday, which he identified before it exploded. Porter had earlier criticized supporters of IRA hunger strikers.

Sands, 27, who is serving a 14-year jail sentence in Belfast's Maze Prison in British-ruled Northern Ireland for firearm offenses, is reported by British officials and his family to be within a day or two of death. His weight has dropped from 155 to 95 pounds in his last two months on a diet of salt and water.

His protest has focused the IRA challenge to the British government, which has consistently refused in recent years to give Sands and hundreds of other convicted IRA prisoners the special treatment they claim as "political prisoners," even though many of them have refused to wear prison clothing and smeared themselves and their jail cells with excrement to further their protests. The concessions would involve inmates being allowed to wear their own clothes, exemption from prison work and freedom of association within the prison and would amount to recognition of the banned terrorist group as a legitimate political movement.

As last minute efforts this past weekend by two representatives of the European Commission on Human Rights to intervene on Sands' behalf failed, tension in Ulster has palpably increased. The IRA has distributed leaflets in Catholic working class neighborhoods here warning residents of violence to come and urging them to prepare for prolonged disturbances.

The four main Protestant paramilitary organizations in Northern Ireland agreed this week to form a joint command called the Ulster Army Council.

Yesterday supporters of Sands staged the largest protest march in the last decade in Northern Ireland -- estimated at between 6,000 and 10,000 -- marching through Catholic West Belfast.

Although British officials say that no new attempt to mediate with Sands is planned, John Hume, a member of the European Parliament and as leader of the mainly Catholic Social Democratic and Labor Party, done of the most respected Catholic politicians in Ulster, has gone to Dublin to hold urgent talks on the crisis with Irish Prime Minister Charles Haughey.

Britian formally adopted a policy of refusing to force-feed hunger striker after the World Medical Convention meeting in Tokyo recommended in 1975 that it should not be a policy of democratic governments. British force-feeding of two young IRA women in 1974 for more than 200 days had caused a public outcry.