The Green party was today hailing a historic general election performance, after Caroline Lucas became the party's first-ever MP by winning the Brighton Pavilion seat.

But while Lucas, the Green leader, said the election showed voters "want something different", her party's overall share of the vote actually dropped slightly nationwide, hovering near a disappointing 1%.

While they fielded a record 300-plus candidates, the Greens opted to concentrate most resources on just three target seats, Brighton Pavilion among them, a risky decision that seems to have left candidates elsewhere squeezed amid tough battles between the big three parties.

Lucas's success in turning a 5,000 Labour majority into a margin of success of just over 1,200, a swing of 8.4%, represents a stunning coup for a relatively tiny organisation.

The 49-year-old MEP – she will now give up her Strasbourg seat – has been the favourite in Brighton for some months, but a resurgent Labour challenge threatened to derail her momentum at the last minute. In the end, the count was sufficiently tight that it was almost 6am when Lucas was finally declared the winner, to shouts of joy and disbelief from supporters in the room.

"For once, the word 'historic' fits the bill," she told the audience to loud cheers. "Thank you so much for putting the politics of hope above the politics of fear. This isn't just a moment when one MP out of more than 600 is elected. It's where a whole political party takes, for the first time, its rightful place in our parliament."

Her victory comes down to a series of factors, including Lucas's own profile as the face of a party which once eschewed traditional leaders, and a strong party organisation in a city with 13 Green councillors. More than 200 Green activists were on the ground yesterday making sure supporters went to the polling stations.

Additionally, when the decision was made to focus Green efforts on Brighton's sizeable population of students and environmentally-minded types, the party was also more ruthless than stereotype would suggest, with the 2005 candidate, Keith Taylor, dumped in favour of Lucas, despite greatly increasing the party's vote. He will now take her European parliament seat.

There was mixed success last night in the other two seats the Greens had talked up, albeit more speculatively, as being potentially winnable. In Norwich South, the 28-year-old Green deputy leader, Adrian Ramsay, increased his vote to 15% but still came fourth as the Liberal Democrats unseated Labour's former home secretary, Charles Clarke. In Lewisham and Deptford, in south London, Darren Johnson saw his share of the vote fall as Labour's Joan Ruddock won easily.

Nonetheless, Lucas claimed her election marked "the start of a new political force in Westminister", saying she would decide which party to support in a hung parliament on a "case-by-case basis".

As well as two MEPs, the Greens previously had representation on the London, Scottish and Northern Irish assemblies, and more than 125 councillors in England and Wales. In last year's European elections, the Greens won an 8.7% share, well above yesterday's overall support.

At this election, the Greens sought to highlight policies such as a "citizens' pension" of £170 a week and heavy spending on health and education above their better-known environmental commitments.

Despite the mixed overall result, the party is clearly thrilled at gaining an MP. Even at 7am, a crowd of almost 200 activists waited outside the seafront Brighton Centre, where the count took place, to mob Lucas as she finally left. "We have done something extraordinary today," she told them as they chanted her name.