The deputy mayor of Mariupol said the temporary halt to hostilities - which started at 9am local time - ended within hours, throwing the planned evacuation of the city into turmoil and forcing residents to seek shelter wherever they could.
Serhiy Orlov said the ceasefire had collapsed amid the Russian onslaught on Mariupol and the eastern city of Volnovakha.
He added: "The Russians are continuing to bomb us and use artillery. It is crazy."
Confirming that he felt compelled on safety grounds to urge people to return, he added: "We value the life of every inhabitant of Mariupol and we cannot risk it, so we stopped the evacuation.”
A 'green corridor' set up to allow people to flee the city was also targeted, forcing many to turn back and seek shelter amid bomb-damaged buildings.
Ukraine's army maintained control of the city but Russian forces repeatedly targeted residential areas with air strikes, cutting off water, power and sanitation.
At 10am local time the Mariupol authorities were forced to announce the evacuation was postponed because of the attacks.
In a statement, they said: "We ask the people of Mariupol to the shelter.
"Due to the fact that the Russian side is not sticking to the ceasefire and continues to shoot Mariupol itself and the outskirts, the evacuation has been postponed."
The International Committee of the Red Cross (ICRC) said its priority was the "safety and wellbeing of all civilians living through terrifying situations in Ukraine".
"We remain in dialogue with the parties about the safe passage of civilians from different cities affected by the conflict," the neutral ICRC said.
It called on those involved in the fighting "to protect civilians and civilian infrastructure" in accordance with international law.
Mariupol, which has a population of approximately 400,000, is a key target for Russia. Seizing it would allow Putin-backed separatists in eastern Ukraine to join forces with troops in Crimea, which Russia annexed in 2014.
Reacting to the ceasefire breach, Russia's defence ministry said civilians had not used the planned escape routes and accused Ukrainian authorities of preventing people from leaving.
Elsewhere, fighting continued in the north and the east of the country, with several cities shelled and reports of hand-to-hand combat.
It was reported that a psychiatric hospital in a town 60km (37 miles) north-west of Kyiv was captured by Russian forces, trapping more than 650 patients.
Ukrainian news platform Hromadske said Kyiv's regional governor confirmed the hospital in Borodyanka had been taken.
"We do not understand how to evacuate these people, how to help them. These are people with certain special needs, they need constant help, many of them have been bedridden for years."
Before Russia announced the ceasefire, Ukraine had urged Moscow to create humanitarian corridors to allow children, women and the older adults to flee the fighting, calling them "question number one".
Russian foreign minister Sergey Lavrov said he was ready for a third round of talks aimed at ending the conflict but accused the Ukrainiane of delaying tactics.
Diplomatic efforts are set to continue as US secretary of state Antony Blinken arrived in Poland to meet the prime minister and foreign minister, a day after attending a Nato meeting in Brussels in which the alliance pledged to step up support for eastern flank members.
In the wake of Western sanctions, Aeroflot, Russia's flagship state-owned airline, announced that it plans to halt all international flights, except to Belarus, starting on Tuesday.
But as the Russian armoured column threatening Kyiv remained stalled, the shelling in Mariupol showed Russia's determination to cut Ukraine off from access to the Black Sea and the Sea of Azov, further damaging the country's economy.
Presidential adviser Oleksiy Arestovich said the situation in the capital was generally quiet on Saturday and Russian forces "have not taken active actions since the morning”.
Ukraine's president Volodymyr Zelensky has pleaded for a no-fly zone and has criticised NATO for refusing to impose one, warning that "all the people who die from this day forward will also die because of you".
NATO has said a no-fly zone, which would bar all unauthorised aircraft from flying over Ukraine, could provoke widespread war in Europe.
In a thinly veiled threat issued on Saturday evening, Russia's foreign ministry warned of sanctions against the UK in retaliation for those imposed following the invasion.
Maria Zakharova, the foreign ministry spokesperson, said Russia will not forget the UK's cooperation with Kyiv, or with what she calls the "ultra-nationalist forces of Ukraine", according to Russian media.
"The sanctions hysteria in which London plays one of the leading, if not the main, roles, leaves us no choice but to take proportionately tough retaliatory measures," she said, adding that British interests in Russia would be "undermined" by Moscow's response.
Meanwhile, the UN World Food Programme has said millions of people inside Ukraine, a major global wheat supplier, will need food aid "immediately".
The UN estimates that 12 million people in Ukraine and four million fleeing to neighbouring countries in the coming months will need aid.
Ordinary Russians will this week start feeling the effect the international outrage over Vladimir Putin’s actions.
Zara, Paypal and Samsung will become the lastest international firms to suspend trading following the invasion of Ukraine.
The clothes retailer's owner, Inditex, will shut all 502 stores of its eight brands, including Bershka, Stradivarius and Oysho, from Sunday, affecting more than 9,000 employees
Payment giant Paypal cited "violent military aggression in Ukraine" as the reason to shut down its services.
Samsung - Russia's top supplier of smartphones - is suspending shipments over "geopolitical developments".
Other global brands, including Apple, H&M and Ikea, have already stopped selling in Russia.