Vladimir Putin was one of around 100 guests at Karin Kneissl's wedding to a businessman in southeastern Austria on Saturday.
Over six million people tweeted on Prince Harry's marriage to Meghan Markle, more than three times the number during his older brother's wedding, social media monitoring firm Visibrain said Sunday.
Residents and medics told Reuters that 20 people attending the celebration were killed and at least 30 injured.
Amelia Thomas, who was among the children caught up in a deadly attack on a pop concert last year, thought it was a joke when her mother said she had been invited to Windsor Castle for the wedding of Prince Harry to his US fiancee Meghan Markle.
When more than 100,000 flag-waving, cheering royal fans gather for the wedding of Prince Harry to Meghan Markle next month, British police will be treating every one as a potential terrorist, prankster or royal obsessive.
Thames Valley Police announced a raft of security measures already in place, including road barriers and number-plate-recognition technology ahead of the big day.
Kensington Palace said all those invited to the May 19 ceremony at St George's Chapel, Windsor Castle, his grandmother Queen Elizabeth's home west of London, would also be invited to the lunchtime reception afterwards.