'Anti-Ukrainian plan'
Kiev insisted that the unrest in the east was part of a wider plan on the part of Moscow to divide Ukraine and seize territory.
"An anti-Ukrainian plan is being put into operation ... under which foreign troops will cross the border and seize the territory of the country," Prime Minister Arseny Yatseniuk said in public remarks to his cabinet. "We will not allow this."
Ukraine's Interior Ministry was quoted by Interfax-Ukraine news agency as saying those detained were suspected of "illegal activity related to separatism, the organization of mass disorder, damage to human health" and breaking other laws.
However, there were signs that Kiev's security operation in the east might not seek immediately to regain all those buildings captured on Sunday.
Russia's Itar-Tass news agency quoted Ukrainian Deputy Prime Minister Vitaly Yarema as saying there would be no storming of the regional authority building in Donetsk on Tuesday.
He said the decision was made after talks in Donetsk with the protesters involving influential and wealthy businessman Rinat Akhmetov, who is from the city.
On Tuesday, about 200 people were gathered in front of the building and a group of National Guard troops were stood to one side. But the situation was calm and there was no sign of any attempt to enter by force.
'Respectful attitude'
Russian Foreign Minister Sergei Lavrov and his Ukrainian counterpart Andriy Deshchytsya met for talks late on Monday.
After the talks, Lavrov stressed the "necessity of a respectful attitude to the aspirations of the inhabitants of southeastern Ukraine."
Lavrov said Kiev must not allow "attempts to react by force to their legal demands for their linguistic, cultural and social-economic rights".
Lavrov called for Kiev to take "urgent measures" to organize a national dialogue, saying it was ready to "support this process along with the European Union and the United States".
AFP-Reuters