The killing today of a taxi driver who was taking a miner to work in the strike-shut coal pits of south Wales has raised the level of violence in the 37-week-old strike and brought condemnation by all political leaders and many striking miners.

The incident also has dramatized the escalating tensions in Britain's mining communities in the past few weeks since the state-run National Coal Board began seeking to attract miners back to work by offering a special Christmas package of wages and bonuses.

The taxi driver, 35-year-old David Wilkie, was killed before dawn when a 3-foot-long concrete post was thrown from a highway overpass onto his taxi, which was being escorted by police motorcycles and a police car.

Police charged Reginald Hancock, 21, and Russell Shankland, 20, both striking miners, with killing Wilkie, United Press International reported.

Wilkie is the first person killed in a deliberate attack since the strike began almost nine months ago. The miner in the taxi was not hurt.

Two miners had been killed previously by accident, one crushed in a huge shoving match between pickets and police and the other hit by a truck trying to move through a picket line. Two other miners have committed suicide, and one working miner died of a heart attack after a mob surrounded his house and threw a brick through his window. About 3,000 other miners and 800 policemen have been injured.

Today's incident, however, appears to have had a marked effect on people on both sides of the dispute. The chief police constable in south Wales called the crime "murder."

Prime Minister Margaret Thatcher, during an official visit to France, described the attack as "calculated malice. No decent thinking person can support a strike sustained by this violence and thuggery. Last weekend and this weekend, we went somehow to new depths of violence," she said. "This is not in the British character, not the British way. It is being done to sustain a strike" that, she said, never should have been called because the National Union of Mineworkers never held a ballot among its members.

Throughout the strike, about a fourth of Britain's 180,000 coal miners have kept working, and in recent weeks the National Coal Board's offer of holiday pay and bonuses has lured as many as 13,000 back to work, according to board figures that the union disputes. But that has also spurred new levels of violence by those seeking to stop this drift back or, as some union officials claim, by outsiders seeking to take advantage of the situation.

Last week, a gang broke into the home of a working miner and beat him while another working miner's home was set afire and the daughter of another worker was stoned in her car.

Today, a mine union official in south Wales, Kim Howells, expressed great sorrow over the killing and said it was totally unjustified. But he said, "it was inevitable that some kind of tragedy would occur because of the way in which the coal fields have been wound up over the past two to three weeks" by the coal board. "The whole policy of trying to break a solid coal field by cajoling and tempting men to come back to work is going to result in tempers and passions being raised in communities that were 100 percent behind the strike," he said.

South Wales is one of the most solid areas supporting the strike, and the miner who was in the taxi today is only one of two men who have crossed picket lines at the Merthyr Colliery, where several hundred are on strike.

Many striking miners said on television that, as one put it, "Whoever has done this has done a lot of damage" to the cause of the strikers. "It's just plain stupid."

At a previously scheduled miners' rally tonight, mine union chief Arthur Scargill said the union "disassociates itself from any acts of this kind which occur anywhere away from the picket lines."

The Associated Press, meanwhile, reported from London:

An alert mail sorter at a London post office Friday intercepted a letter bomb addressed to Prime Minister Margaret Thatcher. The device did not explode, and no one was injured.

A Scottish separatist group called the Scottish National Liberation Army claimed responsibility for sending the bomb in a call to the Press Association news agency.

Bomb experts said the "viable incendiary device" was concealed in a white envelope, 10 inches by 3 inches, addressed to Thatcher at her official residence, 10 Downing St.