Gordon Brown has called on the United Nations to send its envoy to deal with the crisis in Burma, saying that "the whole world is now watching".
The comments come amid reports that police have beaten and arrested demonstrators and have fired warning shots on the ninth day of protests.
The UN Security Council is meeting in New York to discuss the crisis.
The Tories said the Burmese government should be left "in no doubt that the whole world will unite against them".
Attending Labour's annual conference in Bournemouth, Mr Brown said "the age of impunity and over-riding human rights is over".
Calling Burma's government an "illegitimate and repressive regime", Mr Brown said: "The whole issue of sanctions is going to take on a new dimension."
He called for UN envoy Ibrahim Gambari to be sent to Burma, to "make sure that the Burmese regime directly is aware that any trampling of human rights that takes place will have the whole eyes of the world upon them and will not be acceptable in future".
Mr Brown said: "I want to see all the pressures of the world put on this regime now - sanctions, the pressure of the UN, pressure from China and all the countries in the region, India, pressure from the whole of the world."
The European Union was looking at sanctions. he told delegates.
Mr Brown added: "If we could send a message today from this conference that we support Aung San Suu Kyi and all those people fighting for democracy against this illegitimate and repressive regime, then I think that is another sign that the world will not stand by and that this regime is under so much pressure now from all opinion throughout all the world."
The prime minister spoke as several thousand monks and civilians marched in Burma's capital Rangoon.
Analysts fear a repeat of the violence in 1988, when troops opened fire on unarmed protesters, killing thousands.
Earlier, Foreign Secretary David Miliband said Burma's government must listen to the "unanimous international call for restraint" in dealing with protests.
He warned that the ruling junta would be "held to account", with economic sanctions possible.
Shadow foreign secretary William Hague said: "We welcome the UN Security Council meeting to discuss the situation in Burma.
"The barbarous military regime must be left in no doubt that the whole world will unite against them if they use violence against their own people.
"Any attempt to crush the demonstrations must be answered with decisive and united action by the international community."
Lib Dem foreign affairs spokesman Michael Moore said the monks were an inspiration and the Burmese people must be supported "in their struggle against tyranny".
He said Mr Brown was right to call for a security council meeting and said China and India must use their influence on the regime in Burma.
"The situation is critical and fast-changing, but we must make clear that we expect the junta to show restraint and to refrain from further action against the protesters," he said.
"The international community must do more than watch impotently from the sidelines."
The junta broke its silence over the mounting protests late on Monday, saying it was ready to "take action".
Conservative MP for Buckingham John Bercow, chairman of the all-party parliamentary group on democracy in Burma, told the BBC the country's military regime was "despicable".
US President George W Bush has announced a tightening of existing US economic sanctions against Burma.