Burma's ruling military junta has warned it is ready to "take action" against Buddhist monks leading mounting protests, state media have reported.
Brig Gen Thura Myint Maung, minister for religion, warned them not to break Buddhist "rules and regulations" as Rangoon saw the largest march yet.
He blamed the protests on "destructive elements" opposed to peace in Burma.
President George W Bush is set to announce fresh US sanctions on Burmese leaders, the White House says.
The sanctions, which will include a ban on US visas, will be announced during Mr Bush's speech at the United Nations on Tuesday, US National Security Adviser Stephen Hadley said.
The military government has so far showed restraint against the protests.
Monks are highly revered in Burma and correspondents say any move by the junta to crush their demonstrations would spark an outcry.
But there are fears of a repeat of 1988, correspondents say, when the last democracy uprising was crushed by the military and some 3,000 people were killed.
Some monks' representatives had called for the entire country to join them in their campaign to overthrow the government, which began eight days ago.
15 Aug: Junta doubles fuel prices, sparking protests
5 Sept: Troops injure several monks at a protest in Pakokku
17 Sept: The junta's failure to apologise for the injuries draws fresh protests by monks
18-21 Sept: Daily marches by monks in Burmese cities gradually gather in size
22 Sept: 1,000 monks march to the home of Aung San Suu Kyi in Rangoon
23 Sept: Up to 20,000 march in Rangoon
24 Sept: New Rangoon march draws at least 50,000 and 24 other towns join in
Monday saw marches in at least 25 towns and cities, including Mandalay, Sittwe and Pakokku.
Turnout estimates in Rangoon, Burma's biggest city, range from 50,000 to 100,000.
According to state media, the minister for religion spoke after meeting senior members of the Buddhist clergy, whom he warned to control the militant young monks who appear to be leading the current street protests.
In the first public response by the junta to the mass protests, he said action would be taken against the monks' protest marches "according to the law if they cannot be stopped by religious teachings".
No further details were forthcoming, but there was no hint of reconciliation in the government's message, BBC Asia correspondent Andrew Harding reports.
State television said the demonstrations of the past week were being fomented by communists and exiled media and student groups.
Dalai Lama appeal
Our correspondent says Monday's marches are a show of defiance unthinkable just a few weeks ago.
Five columns of monks, one reportedly stretching for more than 1km (0.6 miles), entered the city centre to cheers and applause from thousands of bystanders.
Civilians who joined in included officials from the opposition National League for Democracy (NLD) party, led by Aung San Suu Kyi.
The authorities are likely to be under huge pressure from their close neighbour China to avoid bloodshed and instability, our correspondent notes.
But if the demonstrations continue, he adds, the generals may see their authority ebb away and their options narrow.
The European Union has urged the junta to show the "utmost restraint" in dealing with the protests and to take the opportunity to "launch a process of real political reform".
The exiled Tibetan spiritual leader, the Dalai Lama, has given his backing to the monks' call for freedom and democracy but urged the military not to react with violence to the protests.
UK Ambassador Mark Canning said Burma's leaders were now in uncharted territory and he expressed concern about a possible government counter-reaction.
"That... would be a disaster, although in terms of probability it, I'm afraid, ranks quite high," he told the BBC.
A hard-core group of more than 1,000 of the maroon-robed monks and 400 sympathisers went to Aung San Suu Kyi's street at the end of Monday's march, the Associated Press reported.
They chanted a prayer for peace in the face of the riot police blocking access to her home, where she is under house arrest, before dispersing peacefully.
Monks have been urging Burmese people to hold 15-minute evening prayer vigils.
The organisation that has emerged to lead the protests, the Alliance of All Burmese Buddhist Monks, has vowed to continue marches until it has "wiped the military dictatorship from the land".
The protests were triggered by the government's decision to double the price of fuel last month, hitting people hard in the impoverished nation.
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I'm a University student from Mandalay. I think Mandalay needs a leader as people are very afraid. But they want to get freedom. Now I saw 10,000 monks and a number of other people. Juee, Mandalay
I witnessed the big protests in Rangoon today. I am really sorry for our country and our people because we are under the control of the wicked junta. We haven't got arms, we wish for peace, a better future and democracy. We are hoping that the UN security council will put a pressure on the junta. Kyi Kyi, Rangoon
I saw more than 100,000 monks march to 8 -mile junction, from south to north, I'm not sure where they lead to. This situation could be worse more if junta couldn't control softly. We want some changes in Burma, but we want these changes leading to good ways. I can say surely that killing the monks is insulting the Buddhism religious. Sun, Rangoon
I saw about 40,000 to 50,000 people, including monks, nuns and ordinary people, marching along Prome road. That was at around 3:30pm. The protests will grow bigger day by day and I hope that they are not going to start killing people. We need help to save our people. Mr Tun, Rangoon
It is astounding to see such a great mass of people on both sides of the roads, some clapping and some crying, but all demonstrating their support for the monks and those chanting prayers. It's for sure that all these people showing their support are willing to be part of the mass protest. They do not trust the government though and think that they could be crushed, just like it happened in 1988. But if we are just bystanders, today's rare and momentous events might not lead to the fall of the regime. Kyaw, Rangoon
I am not sure where these protests are going to lead to, but I am sure that it's not at all a good sign. Many people are expecting that there will be a great change coming soon. I am not sure if the monks will be joined by students, workers, or even soldiers. We are very insecure because we don't know what the government is planning to do. There are some news in the government - controlled newspapers that the monks are trying to agitate the public. This can be a big excuse for them to start attacking the monks. I really want some changes in Burma but I am not sure where the change is going to lead us to. I hope there won't be any blood-bath this time like there was in 1988. Soe Soe, Mandalay
We need a little reason to combine all opposition resources into one mass movement. This reason must be a political issue. The current situation can lead to a civil war because hardline junta still holds the power and the opposition might use this opportunity to form an armed struggle. After 1988, many activists, including students, ran to the border and took up the arms against the government. This time we want things to change peacefully, not through a civil war. But if there's no way to avoid the arms struggle, the people will choose it and the conditions in our poor country may become worse. The international pressure, including from China and Russia, is very important for the future of Burma at this moment. Mg Khar, Rangoon
One of the monks who took part in the protests came to us and told us about his experiences. He said: "We are not afraid, we haven't committed a crime, we just say prayers and take part in the protests. We haven't accepted money from onlookers although they offered us a lot. We just accept water. People clapped, smiled and cheered us." The monk seemed very happy, excited and proud. But I'm worried for them. They care for us and we pray for them not to get harmed. Mya, Rangoon
One reason why the generals are hesitating, may not be merely reverence. Spiritually speaking, if one is a Buddhist, one will clearly know that one of Buddhist's big sin is the physical harming of a monk who had taken the vow. Such bodily harm of a monk will carry the worst penalty of sufferance in hell. From the perspective of a devout Buddhist nation, that maybe the reason for the junta's unwillingness to act, hoping the march will fizzle out on its own. Tristan Toh, Singapore