KYIV (REUTERS) - Russian forces used tear gas and stun grenades to disperse a pro-Ukraine rally in the occupied city of Kherson on Wednesday (April 27), Ukraine's Prosecutor General said, as Moscow tightened its grip over the southern region.

Local authorities say Russia appointed its own mayor of Kherson on Tuesday after its troops took over the administration headquarters in the regional capital, which was the first big urban centre to be seized after the Feb 24 invasion.

Some residents have staged occasional anti-occupation rallies and crowds gathered in the centre again on Wednesday, the date Kyiv had said Russia planned to stage a referendum to create a breakaway region like those in eastern Ukraine.

"During a peaceful pro-Ukrainian rally on Freedom Square in the city of Kherson, servicemen of the Russian armed forces used tear gas and stun grenades against the civilian population," the office of Ukraine's Prosecutor General said in a statement.

It said it was investigating the incident, and that at least four people were wounded.

Russia did not immediately comment on the incident.

Ukrainian President Volodymyr Zelensky praised the protests, saying in a late night address that "I am grateful to everyone who has not given up, who is protesting, who is ignoring the occupiers and showing the marginal people who have become collaborators that there is no future for them".

Shortly afterwards, RIA news agency cited a security source as saying Ukraine had fired three rockets at the city centre but Russian occupying forces shot down two of them.

On Tuesday, Russia said it had gained full control of the Kherson region, which is strategically important as it provides part of the land link between the annexed Crimea peninsula and Russian-backed separatist areas in the east.

Russia calls its actions in Ukraine a "special operation" to disarm its neighbour and protect it from fascists. Ukraine and the West say the fascist allegation is baseless and that the war is an unprovoked act of aggression.

Flags and ribbons

Regional governor Hennadiy Laguta on Tuesday said the occupation meant his administration had no choice but to leave.

Thousands of people have fled Kherson to Ukraine-controlled territory, according to the local authorities, who report power and water outages, severe shortages of food and medicines and cellphone network issues in some occupied areas.

Footage shared on social media on Wednesday showed people rushing away from a square filled with drifting smoke. A loud boom rang out and a thick plume of white smoke rose into the air. Coughing could be heard and one person could be seen clasping their hands over their eyes.

Reuters verified the location of the video through the configuration of the buildings which matched street view photography of Kherson.

Two people, who told Reuters they had participated in the rally, said people were waving Ukrainian flags, chanting "Glory to Ukraine" and singing the national anthem when a large number of Russian forces turned up.

Speaking by phone, former factory worker Ihor, 33, said some of the Russians first fired in the air.

"There wasn't panic until they threw the tear gas grenade," said 18-year-old student Mykyta.

Both men, who gave only their first names for security reasons, said they had not seen any sign of the planned referendum being held.

Ihor said those who remained were "depressed".

"People understand that they can't really do anything. The police are everywhere, people are stopped everywhere," he said.

"Many Kherson residents are afraid. All the same, Kherson residents do not give up, they quietly go to rallies. They've hung up flags, blue and yellow ribbons, all in support of Ukraine."