Ukraine's leaders vowed Sunday not to give up "a single centimeter" of territory to Russia as thousands rallied at rival pro- and anti-Moscow demonstrations, and tensions remained high over the deepening crisis in Crimea.
Interim Prime Minister Arseniy Yatsenyuk led commemorations in the capital, Kyiv, for the 200th anniversary of the birth of Ukraine's greatest poet and national hero, Taras Shevchenko.
He told a crowd that the country's "fathers and grandfathers have spilled their blood for this land. And we won't budge a single centimeter."
Other rallies honoring Shevchenko were held in Crimea's key cities of Simferopol and Sevastopol, the eastern city of Donetsk and other areas. Pro-Russia protesters held counter-demonstrations in cities throughout Crimea and eastern Ukraine.
Later Sunday, Mr. Yatsenyuk announced he would travel this week to the United States for high-level discussions on the standoff with Russia over Crimea, a strategic peninsula in southern Ukraine with a Russian-speaking majority.
A White House official confirmed the talks, but no other details were immediately available from Washington officials.
Meanwhile, Russian forces tightened their grip on Crimea despite a U.S. warning to Moscow that annexing the region would close the door to diplomacy.
In the latest armed action Sunday, Russians took over a Ukrainian border post on the western edge of Crimea, trapping about 30 personnel inside.
A Ukrainian military spokesman said Russian forces now control 11 border guard posts across Crimea.
Russia denies it has troops on the peninsula beyond those regularly stationed with its Sevastopol-based Black Sea fleet. Ukraine's much smaller navy is also based in the Crimean port city.
Witnesses say although the soldiers have no insignia identifying them, they are clearly Russian.
Foreign observers have failed to get into Crimea to get a first-hand look at the situation and were forced to turn back Saturday after pro-Kremlin gunmen fired warning shots.
Unease in Crimea continues after Ukraine's acting president, Oleksandr Turchynov, signed a decree Friday canceling a March 16 referendum on Crimea joining Russia. Local authorities in Crimea say the ballot will go forward.
Russian lawmakers have vowed to support Crimea's decision if the Ukrainian region decides to join Russia.