The interior ministry said in a Twitter post that "Polish authorities have foiled an attempt by migrants to force their way into Polish territory to the south of the Kuźnica crossing."
It added that "the situation has been brought under control."
The ministry posted videos showing police, Border Guard officials and troops standing along the frontier fence, behind which a group of migrants can be seen on the Belarusian side, state PAP news agency reported.
Poland's defence ministry tweeted a video of what it said was an attempt to storm the Polish border.
The messages came after a series of reports by news outlets and Polish authorities earlier in the day that a huge group of migrants from the Middle East, accompanied by armed Belarusian officials, had set out towards the Polish border. They were marching with the intention of forcibly crossing into Poland, these sources said.
According to Belarusian journalist Tadeusz Giczan, thousands of migrants moved from the capital Minsk and many were marching in the direction of the border crossing in Kuźnica Białostocka, a village in Poland's eastern Podlaskie province, to "try to storm the Polish border in one large group."
"The storm is planned for tomorrow morning," Giczan said in a tweet in English on Sunday. "I’m afraid it could turn really nasty."
Meanwhile, Jakub Kumoch, a foreign policy aide to Polish President Andrzej Duda, said on social media that members of the Belarusian security services were taking part in the march, while the migrants' children could be used in an attempt to enter Poland, the tvp.info website reported.
"Under such circumstances, a patriot, regardless of their views, stands with the Border Guard and those who are protecting our frontiers," Kumoch said.
Users of the messaging Telegram application have been posting pictures of migrants gathering in front of the Kuźnica crossing.
The marchers were being accompanied by armed individuals with dogs, tvp.info reported.
'The biggest attempt to date to enter Poland by force'
Stanisław Żaryn, the spokesman for Poland's security services chief, tweeted that the the group of migrants, containing "mostly young men," was "under strict control of Belarusian armed units" who were in charge of where the group was heading.
He warned the group would mount "the biggest attempt to date to enter Poland by force."
Polish forces 'prepared for every eventuality'
Deputy Interior Minister Maciej Wąsik tweeted that the migrants "won't go through" the border, adding that Polish security forces were "prepared for every eventuality."
Meanwhile, Piotr Müller, the spokesman for the Polish government, announced that a crisis management team led by Prime Minister Mateusz Morawiecki and including Deputy Prime Minister Jarosław Kaczyński and the defence, interior and foreign ministers, was due to convene at 1 p.m., tvp.info reported.
'Belarus wants to cause a major incident'
In an interview with public broadcaster Polish Radio, Deputy Foreign Minister Piotr Wawrzyk said on Monday morning that "Belarus wants to cause a major incident, preferably with shots fired and casualties."
He added that "according to media reports, they are preparing a major provocation near Kuźnica Białostocka, that there will be an attempt at a mass border crossing."
Wawrzyk told Polish Radio that the "march on the border" showed "the Belarusian regime is keeping up its provocations and resorting to the worst possible tactics."
He said that how the march would end "depends on the other side, because our responses are appropriate to the situation."
Piotr Wawrzyk. Photo: PR24/MS
'We are prepared to defend Polish border'
The Polish defence ministry tweeted: "More than 12,000 Polish soldiers are on duty at the Polish border. We've raised the state of readiness for the Polish Territorial Defence Forces. Together with the services of the Polish Ministry of the Interior and Administration, we are prepared to defend the Polish border."
The Polish 'border is sacred': PM
Polish Prime Minister Mateusz Morawiecki said on Facebook that "the Polish border is not just a line on a map. The border is sacred - Polish blood has been spilled for it!"
The migrant crisis on the Polish-Belarusian border has been mounting since the summer.
Poland and the Baltic states have accused Belarus's strongman leader Alexander Lukashenko of organising a wave of illegal migrants seeking to enter the bloc as part of what officials have called a "hybrid war."
The European Union has accused Belarus of encouraging thousands of migrants from the Middle East and Africa to cross into EU countries via Belarus, as a form of hybrid warfare in revenge for Western sanctions on Minsk over human rights abuses, the Reuters news agency reported.
The EU Commissioner for Home Affairs, Ylva Johansson, visited Poland in late September, agreeing with Warsaw’s arguments that “firm steps” were needed against Belarus, according to officials.
Poland's Prime Minister Mateusz Morawiecki said last month that his country enjoyed full support within the European Union as it worked to defend itself against a migrant influx and a "hybrid war" being waged by Belarus.
In late September, Polish lawmakers voted overwhelmingly to extend a state of emergency in parts of two regions along the country's eastern border with Belarus by two months amid a growing migrant surge.
The state of emergency gives authorities broader powers to monitor and control the movement of people on the Polish-Belarusian border, which is also the eastern border of the European Union.
Source: PAP, tvp.info