Ann Clwyd called for a big anti-war march in London against Russia's bombing of civilians in Aleppo, as MPs debated the humanitarian crisis.
Her call was backed by Foreign Secretary Boris Johnson, who warned that the "wells of outrage are growing exhausted".
Russia says it is targeting terrorists.
The northern city of Aleppo is a key battleground in Syria's bloody five-year civil war.
A UN relief convoy was attacked from the air in September - an action widely attributed to Russia.
Russian President Vladimir Putin's government - which supports the government of Bashir al-Assad - has denied carrying out the action.
On Tuesday, Cynon Valley MP Ms Clwyd told MPs: "I challenge people listening to this now - let's have two million, three million, four million outside the Russian embassy, day after day after day.
"Let's show what we think of their actions in Syria and their refusal to bring peace to that country."
MPs also called for Britain to review its relationship with President Putin's regime and seek tougher trade sanctions against it.
Mr Johnson said: "It is the UK week after week that is taking the lead together with our allies in America and in France, all the like-minded nations, in highlighting what is happening in Syria to a world where, I'm afraid, the wells of outrage are growing exhausted."
He added, in response to Ms Clwyd's protest call: "I'd certainly like to see demonstrations outside the Russian embassy.
"Where is the Stop The War Coalition at the moment? Where are they?"
Before the debate, Plaid Cymru Westminster leader Hywel Williams backed suggestions that the "perpetrators of the repeated bombing of civilians, hospitals and aid convoys" should appear before the International Criminal Court".
"The innocent civilians who have been killed in Syria are victims of war crimes and those who are responsible must face an investigation," he said.
In a statement issued on Monday, the Russian government rejected UK criticism of its actions in Aleppo, questioning the UK's own contribution to military and humanitarian efforts.
Millions of people in Britain should protest outside the Russian embassy over the bombing campaign in Syria, a senior Welsh Labour MP has said.