Irish President Michael D Higgins has expressed his “deep condolences” to the Emperor of Japan at the “appalling” murder of former prime minister Shinzo Abe.

“The fact that he fell victim to this most brutal of crimes whilst actively engaging in Japan’s democratic process of political choice makes today’s events all the more appalling,” he wrote in a letter to Tokyo’s Imperial Palace.

Mr Abe, 67, was shot from behind during a campaign speech in western Japan on Friday and died in hospital.

Police arrested the suspected gunman at the scene of the attack, which shocked many in Japan – one of the world’s safest nations with some of the strictest gun control laws.

In a letter to Emperor Naruhito, Mr Higgins paid tribute to Mr Abe, who was Japan’s longest-serving prime minister.

“In this moment of darkness, we all take comfort in reflecting upon Mr Abe’s distinguished career in public service in Japan, which spans the period both before and after your ascension to the Chrysanthemum Throne.

“Please know also, your Majesty, that Ireland and the Irish people share your sorrow and offer solidarity with you and the citizens of Japan, whom we count among our most long-standing friends and like-minded partners.”

Irish premier Micheal Martin also said he was “deeply saddened” by the “appalling” killing.

“He was a strong and committed democrat, and it is especially shocking that he was murdered while engaged in that most democratic of activities, campaigning ahead of an election,” the Taoiseach said in a statement.

“Putting your arguments before an electorate and asking for their vote is at the very heart of what we believe in as democratic politicians. The attack on former PM Abe is therefore an attack on democracy itself.

“It is all the more shocking that it happened in a peace-loving country like Japan.

“My most heartfelt and sincere sympathies are with the people of Japan, whom PM Abe served with such commitment and distinction; with Prime Minister Kishida and his colleagues across Government; and especially with Mr Abe’s family.

“On this very sad occasion, on behalf of the Government and people of Ireland, I extend our most profound condolences and solidarity.”

Northern Ireland First Minister designate Michelle O’Neill, who met Mr Abe on his visit to the island of Ireland in 2013, said on RTE Radio that Mr Abe’s death was “absolutely shocking”.

“My thoughts are very much with his family and with the people of Japan.

“I had the pleasure of greeting him actually as a minister in the executive at that time and had deputised for Martin McGuinness and I greeted him off the plane.

“It’s truly a shocking event and clearly all of our thoughts are with the devastated family and the devastated people in Japan.”