South Vietnamese President Nguyen Van Thieu has been forced to declare martial law as communist forces, under General Vo Nguyen Giap, have kept up sustained assaults on several fronts - from Saigon in the south to Hue in the north.
Authorities in the North Vietnamese capital Hanoi, described it as, "a more powerful and more continuous offensive" than ever before.
White House intelligence in Washington anticipated attacks over the Tet holiday to celebrate the lunar new year, but they were surprised by their intensity.
Sporadic fighting is still being reported in Saigon but the main hostilities - which began at 1800 local time two days ago - are reported to have ceased.
Casualties
According to US figures, 4,959 Vietcong have been killed and 1,862 captured while 232 American and 300 South Vietnamese troops have been killed with 929 and 747, respectively, wounded.
Last night, a 19-man Vietcong suicide squad blew a four foot hole in the wall of the US Embassy in Saigon and the nearby British Embassy sustained minor damage.
Vietcong forces have also attacked the Vietnam general staff headquarters, Navy headquarters, two police stations and the Philippine Ambassador's residence as well as blowing up the radio station in Saigon.
Communications are in chaos and commercial flights from the airport have been cancelled.
North Vietnamese - Vietminh - troops have reinforced their siege of Khe Sanh, near the demilitarised zone.
Some commentators expect the so-called Tet Offensive will shatter the American resolve and have a similar effect on the US to that on the French after the North Vietnamese victory at Dien Bien Phu in 1954 - which contributed to the Geneva Agreements later that year.
The Hanoi goverment has offered talks and a seven-day truce if the US stops its aerial bombardments.
Documents captured by the Americans show the Vietminh troops have been promised an end to the war by February.
Watch/Listen
Photo of a Vietcong officer being shot by the chief of police in Saigon
Summary justice for the Vietcong's guerrilla tactics
Footage of a Saigon street-battle
In Context
Although technically the North Vietnamese and Vietcong guerrillas were defeated by the Americans, the impact of the Tet Offensive dealt a hard blow to US morale.
Altogether the communist forces had attacked about 90 towns and hundreds of villages, apparently unfazed by the presence of 500,000 US troops.
Spurred on by the anti-war movement at home President Johnson began peace negotiations in Paris later in the year.
The Paris Peace Accords were finally signed in January 1973, but fighting continued until the Saigon government surrendered to the Vietcong in April 1975.
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