David Cameron has said he is "alarmed and shocked" by the series of fatal shootings in Cumbria.
The prime minister told MPs the nation's "thoughts" were with those who had lost loved ones and "everything" would be done to help them.
Labour leader Harriet Harman offered her "deepest sympathies" to the families of those killed and injured.
The police later said that at least 12 people had died in the attacks.
The body of 52-year old Derrick Bird, who is suspected of carrying out the attacks, has been found and he is believed to have taken his own life.
Mr Cameron began his first prime minister's questions since taking office by expressing his shock at the events and said the government would make sure efforts to deal with the tragedy were properly co-ordinated.
"The government will do everything it possibly can to help the local community and those affected and to keep the House informed," Mr Cameron said.
"When lives and communities are suddenly shattered in this way, our thoughts should be with all those caught up in these tragic events, especially the families and friends of those killed or injured."
Ms Harman offered her condolences to those affected by the "dreadful" events and her "strong support" to the police and the emergency services as they deal with the incident.
The BBC News Channel's Laura Kuenssberg said news of the fatal shootings meant there was a sombre atmosphere in the Commons for Mr Cameron's first question time appearance - the first by a Tory premier since 1997.
As he made his remarks, Mr Cameron was flanked by Deputy Prime Minister Nick Clegg and Foreign Secretary William Hague.
With his party now a member of the coalition government, the Liberal Democrat leader did not ask any questions during the session.
Ms Harman also pressed the prime minister on his response to the Israeli attack on a Gaza aid convoy and the coalition's policies on tax breaks for married couples and extending anonymity to those accused of rape.
Commenting on the shootings after prime minister's questions, Home Secretary Theresa May said events in Cumbria were "very serious and tragic".
Ms May said: "Our thoughts are with the families and friends of the victims of these shootings.
"It's a terrible incident that has taken place in Cumbria today but I would like to pay tribute to the way in which the police and emergency services have worked very closely together to deal with this incident."
She said she would not comment on the details of the police investigation but would make a full Commons statement on Thursday.
Ms May added: "In the meantime, I want the people of Cumbria to know that they have my support and are in my thoughts."