By the time the final brace was welded to the immense metallic fortress in the centre of Seoul, the country knew that this was no longer just about an unpopular policy to resume imports of American beef.

The new presidency of Lee Myung Bak is in meltdown and his closest political allies — including the Prime Minister — resigned yesterday en masse. The sight of a 20ft “Berlin Wall” of shipping containers blotting out all view of the presidential residence revealed a Government rattled to the core.

“June the 10th is rat-catching day,” bellowed protest leaders through vast loudspeakers positioned in front of the graffiti-plastered fortress. “It is the day the Korean people become the cat that destroys Lee Myung Bak.” As the demonstrators swelled in numbers and vehemence, Catholic nuns rubbed shoulders with leather-faced steel workers. Teachers’ unions stood side by side with truant-playing students. Mothers with babes in arms chanted for freedom of the press, while political cartoonists called for cheaper fuel.

Fearing a repeat of the weekend’s violence, South Korea’s national police raised their state of alert to the highest level and readied themselves for a night of rioting. Light skirmishes between supporters and opponents of Mr Lee had already begun by mid-afternoon.

South Korea is no stranger to street protests but as night fell over the capital, the hundreds of thousands of demonstrators who had marched to the heart of the city knew that this was no normal rally. The oratory was much farther to the political left than anyone has heard for decades. The list of grievances — from the privatisation of state utilities to the minimum wage and a proposed “grand canal” across the country — revealed a society profoundly ill at ease with the Government it recently elected.

Dozens of other interest groups and labour unions told The Times that the US beef issue was a convenient pretext for a bigger protest.

“The people of Korea have many scores to settle with this Government,” said Kim Duk Yeop, one of the rally organisers. “Today is where they all come to the surface.” Everyone who joined the march yesterday — including those not even born in 1987 — knew the political significance of the date. It was timed to coincide with the 21st anniversary of the huge pro-democracy protests that shook the authoritarian regime of the time to its foundations and paved the way for the country’s first free elections.

The sight of the massive steel wall, erected by the authorities, and ranks of riot police has played to the protesters’ sloganeering that Mr Lee has “taken Korea back two decades” in terms of democratic rights. That atmosphere was enriched with nostalgic allusions to the riots of yesteryear. Members of one group of demonstrators who had been students in 1987 said that the day would not be complete “without the smell of teargas burning our nostrils”. At one point in the evening the speaker on the main stage was interrupted by the news that the presidential website had crashed. A mighty roar rose into the night, followed by a live rendition of Morning Dew — the anthem of the 1987 protesters and a song that left their 2008 counterparts in tears.

For the growing forces ranged against the President, the resignation of his Cabinet appeared to be a triumph. The resignations plunge Mr Lee’s three-month leadership into a crisis that some believe he may not survive. Since February Mr Lee has weathered a breakdown in relations with North Korea, a scandal that claimed the patriarch of Korea’s most famous company, Samsung, and a spiralling inflation menace that now threatens to derail the economy.

The hysteria over the feared dangers of US beef has proved the most incendiary. Despite his landslide victory, the popularity of the conservative, pro-business leader has fallen farther and faster than any of his predecessors.

The “mad cow” protests mask a strong mistrust of Mr Lee and his plans to revive the economy. For the past six weeks he has faced widespread protests over plans to restart imports of American beef, suspended five years ago amid concerns over BSE. The vehemence of recent protests last month was enough to delay Mr Lee’s plans to begin US beef imports, which are now being tweaked to make them appear more acceptable. The restarting of beef imports is part of a wider free-trade agreement struck between Seoul and Washington in the last days of Mr Lee’s now widely despised predecessor, Roh Moo Hyun.

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