Labour leader Ed Miliband said the killing of a man in Woolwich is a "truly appalling murder".
Mr Miliband, cutting short a Germany visit to return to the UK, said his party offered the Government " complete support".
He said: "This is a truly appalling murder which will shock the entire country.
“All of my thoughts are with the family and friends of the victim.
“The British people will be horrified by what has happened in Woolwich. Theywill be united in believing that this terror on our streets cannot be allowedto stand.
“The Labour Party will offer the Government our complete support in establishing the facts of what happened and ensuring that those responsible face the full force of British justice."