The US secretary of state, Hillary Clinton, has condemned the violent loyalist protests this week that have forced an MP to flee her home in east Belfast, as well as the continued terrorism of republican dissidents.

During her visit to Northern Ireland on Friday as part of a four-day tour of Europe, Clinton expressed support for Naomi Long, the Alliance party MP for Belfast East.

Long was visited by police officers in the early hours of Friday morning to warn her of a death threat against her and advise her to move out of her home.

The Alliance party has been subjected to arson attacks on its offices and the homes of councillors since Monday's vote at Belfast city hall to end the practice of flying the union flag 365 days a year.

Alliance councillors, who hold the balance of power on Belfast city council, forced through a compromise motion that would allow the union flag to fly on designated days only – a move that infuriated hardline loyalists and sparked a week-long spell of rioting in Protestant areas.

Long, a former lord mayor of Belfast, said: "I refuse to allow this threat to stop me from delivering the valuable constituency service which I have developed since first elected in 2001, a service to all of the people of east Belfast.

"This is not an attack on an individual or on a party, but a wanton attack on the democratic process. It is long past time that this vicious campaign of intimidation and violence was brought to a permanent end."

Commenting on the street disorder and intimidation directed against the non-sectarian Alliance party, Clinton said: "The work of peace not complete. I condemn the attacks this week. There will always be disagreements in democratic society but violence is never an acceptable response."

She made her remarks while meeting the first minister, Peter Robinson, and the deputy first minister, Martin McGuinness, at the Stormont assembly. Robinson has issued an appeal for the disturbances connected to the union flag dispute to end.

However, there are further loyalist protests planned for this weekend, with a demonstration outside Belfast city hall on Saturday. A casualty of that protest has been a march from Belfast's Royal Victoria hospital to highlight the need to retain paediatric cardiac surgery. The Children's Heartbeat Trust has cancelled its protest due to fears of trouble erupting in the city centre on Saturday afternoon.

Clinton's eighth visit to Northern Ireland also coincided with the detention of four suspected senior republican dissidents in Derry and the discovery of a new type of mortar bomb during the arrest operation on Thursday night.

The bomb – known as an explosively formed projectile – is similar to devices used by Islamic insurgents to kill British and US troops in Afghanistan and Iraq, according to the Police Service of Northern Ireland (PSNI). Chief Superintendent Stephen Martin said the weapon was capable of piercing armoured vehicles and could kill those inside.

It was discovered in a car stopped by a police patrol in the Creggan area of Derry on Thursday night. Four men in their 40s were arrested, three inside the car and another nearby.

Praising the police for intercepting the weapon, Ronnie McKeegan of the Ulster Unionist party said: "If the bomb had reached its destination I dread to think of what the consequences would have been.

"I would like to congratulate the PSNI for their prompt actions in the city which undoubtedly saved lives and property from being destroyed."

The four men have been taken to Antrim police station for questioning.