(CNN) When a British national inspired by Islamic State plowed through crowds on London's Westminster Bridge on March 22, killing four and injuring many others, acts of solidarity with the UK took place around Europe.

That night, the Eiffel Tower went dark in Paris, an all-too familiar act of national mourning in France in the wake of the last few years of terror attacks there. In Germany, the UK's Union Flag was projected onto Berlin's Bradenburg Gate in another public demonstration of solidarity from one state to another.

Yet as Russia mourns the victims of a terrorist attack that killed 14 and injured 39 at a St. Petersburg metro station on Monday , the response from many European neighbors have been notably lackluster. The absence of prominent memorials has prompted many -- mostly Russians -- to rally around the victims on social media using the hashtag #PrayforStPetersburg.

The mayor of Paris, Anne Hidalgo, has turned off the Eiffel Tower's lights a handful of times in the past -- not just in response to domestic or European terror attacks, but also as an act of solidarity for the residents of war-ravaged Aleppo. On Tuesday afternoon, Hidalgo's office confirmed to CNN that they would again extinguish the lights of their world-famous landmark at midnight for the victims of the St. Petersburg attacks.

But many still rue the international response. Blogger Cristiano Alves tweeted, "It's curious how differently from Paris terror, Facebook did not create any filter with Russian colours," referencing the social media giant's response to the Parisian terror attacks, which allowed users to temporarily add the French flag to their profile picture.