The patrols began after an apparent standoff between the rebel region's self-proclaimed leader Igor Plotnitsky and the interior minister, who's been accused of seeking to destabilise the war-scarred city.
Men wearing white ribbons on their arms blocked access to several administrative buildings in the city, including those housing the government and parliament, and defied the self-proclaimed leadership's calls to disperse, an AFP journalist reported from the scene.
"The cordon will remain in place until the morning for sure," an armed man said, adding military drills were under way in Lugansk.
Several buses with armed men and an armoured personnel vehicle were seen nearby.
According to video footage broadcast by a local TV channel earlier on Tuesday, one of the men said that "planned joint exercises" had been ordered by the interior ministry.
The patrols started after Plotnitsky, leader of the self-proclaimed Lugansk People's Republic, sacked the rebel region's interior minister on Monday.
In a statement released Tuesday, Plotnitsky accused "the illegitimate leadership of the interior ministry" of seeking to destabilise the situation and called on Lugansk residents to remain calm.
"Right now the leadership of the Lugansk People's Republic is doing everything in its power to secure the safety of residents," he said.
"I can confidently say that attempts of certain people to remain in power at the cost of destabilising the situation in the country are futile and will soon be completely neutralised."
The sacked minister, Igor Kornet, for his part refused to step down, while employees of the interior ministry released an open letter saying they would like to continue working under his command.
The rebel region has been blighted by leadership squabbles and several senior rebels have been assassinated.
A conflict between Russia-backed fighters and Ukrainian troops has killed more than 10,000 people since 2014, when pro-Russian forces declared parts of eastern Ukraine independent following Moscow's annexation of Crimea.
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