Exiled former Pakistan Prime Minister Nawaz Sharif has pledged to go home soon and contest elections, in a bid to oust President Pervez Musharraf.
Mr Sharif was deposed in a coup
He was speaking in London soon after Pakistan's top court ruled against the government and said he could return.
Mr Sharif, who left Pakistan after he was deposed by Gen Musharraf in a 1999 military coup, said it was "the beginning of the end" for his rival.
Correspondents say Mr Sharif could still face charges if he goes home.
Jubilant supporters of the exiled politician cheered Thursday's ruling in Islamabad and called on Gen Musharraf to stand down.
Mr Sharif told the BBC: "Dictatorship and democracy don't go together. One will have to go.
"It is dictatorship which will have to go. The sooner Musharraf understands this, the better it is for him and the country."
He said he was not afraid of facing charges upon his return to Pakistan.
"I'm not scared," he said. "If Musharraf wants to fabricate cases against me, let him do that. I'll face them."
Mr Sharif was sentenced to life in prison for tax evasion and treason among other offences and went into exile following the coup eight years ago.
The authorities said Mr Sharif had promised to stay out of Pakistan and away from politics for 10 years in exchange for his freedom and exile.
But Chief Justice Iftikhar Mohammad Chaudhry said in his judgement on Thursday: "(The Sharifs) have an inalienable right to return and remain in the country as citizens of Pakistan."
Mr Sharif's brother, Shahbaz, another politician, was also exiled in 2000.
The Pakistani government said it accepted the ruling but hinted the Sharifs might face legal action on home soil.
Attorney General Malik Mohammed Qayyum said: "Let them come and the law will take its own course."
MUSHARRAF UNDER PRESSURE
9 March: Musharraf suspends chief justice for "abuse of power". Lawyers protest
April: Protests grow, amid clashes with police
12 May: 34 people die as rival political groups clash in Karachi
11 July: 102 people die when army storms radical Red Mosque in Islamabad
July-Aug: Sharp rise in suicide attacks by pro-Taleban militants
20 July: Supreme Court reinstates chief justice
9 Aug: Musharraf rejects emergency rule
23 Aug: Supreme Court says exiled ex-PM Nawaz Sharif can return
Mr Sharif, who still officially heads his faction of the conservative Pakistan Muslim League party from exile, was prime minister twice.
He leads the main part of a six-party religious opposition alliance which is committed to removing Gen Musharraf from power.
Thursday's verdict came at a difficult time for Gen Musharraf, as he is expected to seek re-election later this year.
There has been speculation in Pakistan that he might seek a power-sharing deal with another former prime minister, Benazir Bhutto.
Ms Bhutto, who leads the Pakistan People's Party (PPP), left the country in 1999 amid allegations of corruption.
Her party, which is not part of Mr Sharif's opposition alliance, welcomed Thursday's ruling.
Senior PPP leader Raza Rabbani told the BBC: "When (Ms Bhutto and Mr Sharif) are both back, democracy will be strengthened and dictatorship will be at end."