Flags flew at half-mast over the Kremlin and other official buildings and entertainment programmes were shelved as Russians asked how so many people lost their lives and property in the catastrophe in the southern Krasnodar region.
More than 25,000 people lost part of all of their belongings in the flooding, which overwhelmed the town of Krymsk after torrential rains and also caused significant damage in the neighbouring cities of Gelendzhik and Novorossiisk.
Prime Minister Dmitry Medvedev cancelled a planned meeting over Russia's hosting of the Olympics in 2014 -- in the city of Sochi in a different area of the Krasnodar region -- to review the disaster, official media said.
The embattled local governor described the floods as a "great surprise" but both pro-government and opposition newspapers showed rare unanimity in saying the authorities had badly failed to provide locals with sufficient warning.
"The tragedy of Krymsk was a perfect demonstration of what slovenliness and hoping against hope brings about," said the pro-Kremlin Izvestia daily in a scathing assessment of the official reaction.
The Vedomosti daily said that flooding in the Krasnodar region was in no way a novelty and authorities were well aware of the risk, particularly after deadly floods in the summer of 2002 that also hit Krymsk.
"The catastrophe shows up the inability of the authorities to protect the population from natural disasters," said the opposition-inclined economic newspaper.
"People were not evacuated and were not warned about the threat," it said.
The staunchly pro-government Komosmolskaya Pravda asked simply in a stark headline over a picture of the Krymsk devastation: "Why so many dead?"
It noted that residents had received warnings about the severe weather through SMS messages and also information on the news ticker of local TV. "But, as the inhabitants of Krymsk say, most people knew nothing about this."
The Moskovskiy Komsomolets newspaper said tersely: "The Krymsk catastrophe could have been foreseen and averted.
The governor of the Krasnodar region Alexander Tkachev however described the flood as a "great surprise" and claimed that nothing could have been done to avert it.
"This is the same kind of catastrophe as an earthquake. What can be done? Man here can do nothing against this, he has no chance," he told Izvestia.
President Vladimir Putin, facing the most severe crisis since he returned to the Kremlin in May, ordered an inquiry to explain the massive death toll after viewing the disaster area from a helicopter.
The force of the water was so ferocious that many residents said they suspected the floods were caused by a release of water at a local reservoir on the Neberdzhai River.
Putin, wearing a black shirt, was shown on television at the weekend grilling officials about whether an emergency release of water was possible.
"Where did the water come from?" Putin asked government officials in televised remarks. Governor Tkachev swiftly replied: "It was raining."
The day of mourning also remembers 14 Russian pilgrims who were killed when their bus overturned in Ukraine on Saturday.
The dark weekend for Russia recalled other summertime catastrophes such as deadly wildfires in 2010.
At least 171 people died in the flooding disaster, with the district around Krymsk the worst hit, with rescue teams finding 159 bodies including those of three children, police said.
Krymsk is about 200 kilometres (120 miles) northwest of the Black Sea resort of Sochi where Russia will host the 2014 Winter Olympic Games.