An incendiary device in a package sent to Prime Minister Margaret Thatcher's office at 10 Downing Street burst into flames today as it was being examined, slightly injuring the security official who was handling it.

Police later disclosed that four similar devices were sent today to opposition party leaders in Parliament and a government minister but were intercepted as the mail was being sorted for delivery.

Notes signed by a previously unknown group, the Animal Rights Militia, were found on the packages. However, the Irish National Liberation Army, a terrorist group in Northern Ireland, also claimed responsibility for sending the bomb to Thatcher.

In Belfast, two incendiary bombs were discovered and a dozen hoax calls were reported, all thought to be connected to the first appearance by Northern Ireland Secretary James Prior before the new Ulster consultative assembly. In his speech Prior expressed the government's determination to combat terrorism and said hundreds of police would be added to the local force.

Police believe the incident at Downing Street probably was not related to the continuing Northern Ireland violence. The use of letter bombs has become standard among Britain's terrorist organizations.

At Downing Street, the light package, eight by four inches, was delivered in the midday mail and tagged as suspicious by messengers. Peter Taylor, the long-time manager of the prime minister's office, was inspecting the package when it ignited, burning him slightly, police said.

Thatcher was in another part of the building and was unaware of the danger.

Later, Thatcher appeared at the House of Commons and deplored the violence. Security on incoming mail was tightened in all government offices.

Labor Party leader Michael Foot, Liberal Party leader David Steel, Social Democratic Party leader Roy Jenkins and Timothy Raison, the minister responsible for animal legislation, were the others targeted unsuccessfully today for letter bombs.

Britain's many traditional animal-rights groups issued statements tonight deploring the use of a firebomb. The British Union for the Abolition of Vivisection said the group would condone civil disobedience to protect the rights of animals, "but we are against damage to life because we respect all life, animal and human."