The Pakistani government says a planned march by opposition leader Benazir Bhutto from Lahore to the capital, Islamabad, is illegal.
Benazir Bhutto returned from self-imposed exile last month
A spokesman said that Ms Bhutto would not be allowed to break the law.
The march, due to start on Tuesday, is part of her campaign against President Pervez Musharraf's emergency rule.
Commonwealth foreign ministers meeting in London gave Pakistan until 22 November to lift its emergency rule or face suspension from the body.
Other steps they want Mr Musharraf to take include:
The United States has been pushing for Gen Musharraf to accept Ms Bhutto into a power-sharing deal to help shore up his war against Islamist extremists.
- stepping down as army chief
- releasing all detained political party activists
- lifting all media restrictions
When Ms Bhutto returned to Pakistan from self-imposed exile last month, more than 130 people were killed in a twin bomb attack on her convoy.
Gen Musharraf has promised elections by 9 January
But she said she had to go ahead with the "Long March" from Lahore to Islamabad to put pressure on Gen Musharraf.
"I know it is dangerous but what alternative is there when the country is in danger?" she said in Lahore.
Later on Monday Deputy Information Minister Tariq Azim made clear that the march would not be allowed to take place.
"All processions, rallies, political gatherings at present are outlawed," he told the Associated Press (AP) news agency. He told AP and other media that Ms Bhutto would have to face the consequences if she broke the law.
Gen Musharraf promised on Sunday to hold parliamentary elections by 9 January.
But Ms Bhutto insists that that is not enough and that she cannot accept the continuing state of emergency imposed on 3 November.
When asked on Monday if she was saying no to more talks with Gen Musharraf she replied: "Yes, we are saying no."
When asked if this marked a change in policy she said: "Yes it is a change, it is a change from my party's past policy."
The BBC's Paul Reynolds says few diplomats doubt that Ms Bhutto will resume negotiations with Gen Musharraf if and when the moment comes.
On Friday she was placed under house arrest for the day to stop her holding a rally in the city of Rawalpindi.
Her comments come as Commonwealth foreign ministers gathered in London to discuss possible suspension of Pakistan from the organisation.
The extraordinary session was called after President Musharraf imposed the state of emergency.
When Gen Musharraf seized power in 1999, Pakistan was suspended from the Commonwealth for five years. The Commonwealth's initial demand that he take off his army uniform has still not been heeded.
Ever since he imposed the emergency last week, Gen Musharraf has faced insistent demands from Britain and United States to restore the constitution and reverse the clampdown on pro-democracy activists.
But at a news conference on Sunday he hit back at his critics.
He promised to hold elections on time but he was unapologetic about the state of emergency, saying it was needed to combat Islamist militants and suggesting it could still be in place during the vote.
Gen Musharraf told journalists that despite criticism, so far his Western allies had shown understanding about "the ground reality in Pakistan".
He also made it clear that he would not step down as army chief until a new Supreme Court validated his presidency.
And he refused to restore those judges dismissed under emergency measures.
He said they might have derailed democracy by ruling that his new presidential term was unconstitutional.
Meanwhile, the Pakistani government has warned that it has sufficient retaliatory capacity to defend its nuclear weapons.
It was reacting to a report in the Washington Post which said the US had made contingency plans to intervene to stop Pakistan's nuclear weapons falling into the wrong hands.
The Washington Post quoted unnamed US intelligence officials as saying such plans included contingency action to stop any possible raid on Pakistani nuclear stores and to move the weapons to another location.
Dismissing the report as irresponsible conjecture, the Pakistani foreign ministry said there was no risk of the weapons being taken by any group.
If another country tried to intervene, Pakistan was ready and able to defend its nuclear arsenal, the foreign ministry said.