Ukraine's prosecutors see calls for federalization at allies in the Luhansk region as separatist.

"The demonstrators' speeches, insignia and federalization slogans, as well as calls to hold a local referendum, are explicitly separatist. The regional prosecutor's office sees the unrest in the Luhansk region as illegal and it entered information on them into the database of pre-trial inquiries to be controlled by the Luhansk regional prosecutor's office," the Prosecutor General's Office said in a statement on Monday.

The Luhansk regional law enforcement services reported unauthorized rallies, organized by small groups of unidentified citizens, on April, 21, the statement says.

The so-called "people's assemblies" were held on Monday in different parts of the Luhansk region at which delegates to the regional people's assembly were elected.

Several thousand people have gathered for what local pro-federalization activists call a "people's assembly" in Luhansk in front of the regional security service's headquarters, captured by those who support Ukraine's federalization on Monday, an Interfax correspondent reported from the scene.

The flag of the self-proclaimed Donetsk People's Republic was set up on an improvised stage.

The demonstrators, who want Ukraine to become a federation and to join Russia, are chanting: "Russia! Referendum!"

"We are for a federation integrated with Russia," an activist said to the audience's cheers.