British army in Nato Black Eagle exercise
The British army is taking part in its largest armoured deployment to Eastern Europe in six years, in a show of support, the Ministry of Defence says.
More than 1,000 military personnel and 100 armoured vehicles have joined Nato's Black Eagle exercise in Poland.
The MoD said it showed the British army's ability to deploy "at short notice anywhere in the world" in support of its allies.
The exercise comes amid tensions with Russia over its actions in Ukraine.
The British army's "lead armoured battle group" of 1,350 troops and 100 armoured vehicles from the 3rd UK Division are taking part - making it the UK's largest commitment to the region since 2008.
Jonathan Beale, BBC defence correspondent
It's been almost a decade since British tanks were last seen rolling across the plains of Eastern Europe.
In northern Poland they have been back in force in a scene reminiscent of the Cold War era.
This is the furthest east in Europe British tanks have been.
And it's a direct response to the actions of a near neighbour - Russia's intervention in Crimea and eastern Ukraine.
But Defence Secretary Michael Fallon says it's not Nato but President Putin who's fuelling talk of a new Cold War by violating international law.
Britain is planning more exercises like this, as part of a Nato response to reassure allies on its eastern flank.
The irony is that it comes at a time when British forces that were stationed in Germany to deal with the threat of the former Soviet Union are slowly pulling out.
As part of the exercise, The King's Royal Hussars (KRH) have operated 20 Battle Tanks and conducted live battlefield scenarios alongside Polish counterparts.
Meanwhile the 1st Battalion, The Royal Welsh have provided troops on the ground.
The MoD said the exercise marked "a significant demonstration of the UK's support to the region" as well as "the British army's ability to deploy an armoured battle group at short notice anywhere in the world in support of the nation's allies".
Exercise Black Eagle follows the early departure of Russian President Vladimir Putin from the G20 summit of world leaders in Australia on Sunday.
Prime Minister David Cameron said those at the summit had sent a "very clear message" to President Putin about the West's readiness to ramp up sanctions unless he took action to defuse the current crisis in Ukraine.
When the UK announced its commitment to the exercise in July this year, Defence Secretary Michael Fallon criticised Russia's "illegal annexation of Crimea and its destabilisation of eastern Ukraine".
He said: "It is right that Nato members and partners demonstrate our commitment to the collective security of our allies in Eastern Europe."
The exercise will finish later on Friday when the Chief of the General Staff, Commander Land Forces Lieutenant and high-ranking Polish officials will be shown a display of battlefield manoeuvres.