Kherson residents have been boarding boats in an apparent evacuation as Russia claimed Ukraine is planning a major attack and President Putin declared martial law in four occupied regions.
Russian media showed people in the city boarding ferries to cross from the right bank of the Dnipro river to the left.
The Moscow-backed head of the southern Kherson region, Vladimir Saldo, said about 50,000-60,000 people would be evacuated over the next six days and claimed Ukraine was preparing “a large-scale offensive”.
He also banned civilians from entering the region for a week.
Ukraine has called the evacuation a propaganda stunt and scaremongering.
Kherson residents apparently received a text message telling them “to evacuate quickly” because “shelling of residential blocks by the Ukrainian armed forces is expected”, according to an unverified screenshot on social media.
Kirill Stremousov, deputy head of the local administration, also made a video appeal.
“I ask you to take my words seriously and to interpret them as a call to evacuate as fast as you possibly can,” he said.
“We do not plan to surrender the city, we will stand until the last moment.”
Russian forces in the area have been pushed back about 13 to 20 miles (20-30km) in the last few weeks and risk being pinned back against the Dnipro river.
The head of the Ukrainian president’s office, Andriy Yermak, posted on Telegram: “The Russians are trying to scare the people of Kherson with fake newsletters about the shelling of the city by our army, and also arrange a propaganda show with evacuation.”
Ukraine’s recent counterattacks have won back considerable territory in the south and east and the new Russian commander in Ukraine, General Sergei Surovikin, has called the situation in Kherson “tense” and “difficult”.
A news blackout in southern Ukraine has added to speculation about a major counterattack, with Sky News analyst Professor Michael Clarke saying it means “something big is going on”.
Many in Kherson city, which used to have a population of about 280,000, have already fled to other areas of Ukraine or abroad.
Kherson is one of the regions Russia now claims as its own after holding referendums that were condemned as a sham by the West.
President Putin on Wednesday declared martial law in those regions – which also include Donetsk, Luhansk and Zaporizhzhia.
Martial law normally means the military is in charge instead of a civilian government and that normal laws are suspended, but so far precise details are unclear.
The Russian-installed heads of the four regions were also given extra emergency powers – again the specifics were not disclosed – and President Putin ordered the creation of territorial defence forces.
A month ago, the president called up 300,000 military reservists, a move that prompted long queues at borders as people tried to avoid being drafted.
There have been fears Mr Putin could shut Russia’s borders to stop fighting-age men from leaving but he stopped short of that during Wednesday’s meeting with his security council.