But shares have, of course, risen dramatically since the trauma of 1987. And in percentage terms the slide was less than occurred during the crash and pales against plunges since the horrific meltdown. In October five years ago Footsie slumped just over 4 per cent against 2.51 per cent yesterday.
Futures action on both sides of the Atlantic did some damage. Worries about interest rates were another influence. Vague indications highly geared maverick hedge funds may be cutting their London positions were also unsettling.
But the dramatic slide seemed to be more the snowballing result of a series of unrelated and undramatic happenings and views than any single, identifiable factor. Trading was not heavy and there was no panicky selling.
Blue chips went into retreat from the first bell although manoeuvring ahead of the Footsie futures expiry allowed shares to more or less hold the line in early trading. Once the futures action was completed worries about New York sent Footsie steadily lower.
When New York opened sharply down a relatively gentle slide steepened. And it seemed to gather its own momentum. Even a modest transatlantic rally was brushed aside.
So the index closed at 4,865.8 which, illustrating just how strong blue chips have been, is the lowest for only three weeks.
There is, of course, a pressing desire in some quarters to get Footsie lower. Many believe there is a need for a correction and blue chips are hopelessly overvalued.
A Friday afternoon in the holiday season with many market men, including experienced players, away from their offices presented an ideal environment for what amounted to a rout in a vacuum.
The stories going the rounds ranged from big sell programmes with institutions allegedly switching into second liners and talk a big investment house was dumping stock ahead of a downbeat review of blue chips.
There was, of course, some profit taking.
Many market men confessed they were puzzled by a crash without reason or substance. Said one: "It's something of a mystery. Nothing has happened to spook the market. Many feel a correction is necessary but the nature and speed of the fall is astonishing."
It will be interesting to see whether the market adopts a more resilient attitude on Monday. If further weakness occurs - and US interest rates could go up next week - then the great bull run could be over.
Amid the mayhem second and third liners kept their cool. The FTSE 250 index edged ahead; so did the FTSE SmallCaps index. They were. however, not immune from the demoralising behaviour of their peers and finished below their best. Among supporting shares ending higher were Greenalls, the hotels and pubs chain, 10.5p to 487.5p, and Psion, the computer group, 15p to 376.5p.
Financials, which led the blue-chip charge, took a battering. HSBC, ruffled by higher Hong Kong interest rates and the biggest share fall since the Chinese took over, fell 166.5p to 2,167.5p. It alone accounted for 16 points of Footsie's fall.
Associated British Foods, on talk it could lead a break-up bid for struggling Dalgety, lost 38.5p to 498.5p and BSkyB, as Sam Chisholm's swan song failed to please, lost 33p to 437p.
On such a day blue chip flyers were few and far between. National Grid, with a 2.4 per cent advance to 256p, topped the pile. Significantly, BTR made further headway, up 1p to 215p.
Alliance & Leicester reflected solid interim figures and talk it is near to making an acquisition with an 8p lift to 620.5p.
Eidos, the computer games group, crashed 102.5p to 447.5p following a widening first-quarter loss and allegations regulators were investigating share dealings.
Proving a day is a long time in the market Graystone, an engineer, said bid talks were off. On Thursday it issued a statement that the negotiations, started a month ago, were continuing. With the company adding that figures would not meet market hopes the shares more than halved to 39p.
A warning from ABI Leisure, the caravan group, that a forecast two weeks ago of profits of pounds 4.5m was wrong and the group would suffer a loss, knocked the shares 27p to 38p; Portmeirion Potteries was smashed 65p to 430p on a cautious trading statement.