The DUP has been accused of playing “hokey cokey” over its decision to halt all but exceptional Executive meetings at Stormont.
Standing over his decision to pull out of the five-party coalition, UUP leader Mike Nesbitt said the DUP was sending out mixed messages.
He said: “I think they are being a little bit more confused.
“I think it is a bit of hokey cokey they are playing.
“We at least were a bit more definite. We said business would not be as usual.
“This is the first day in the history of parliament in Northern Ireland that an Assembly has been in session without an Ulster Unionist Party in government.”
Although the UUP would embrace the forthcoming talks process, Sinn Fein and the police must be on the “same page” with regards to the status of the IRA, Mr Nesbitt said.
Sinn Fein said it was focused on fighting austerity and would not be detracted by unionist rivalry.
Newry and Armagh MLA Conor Murphy said: “We are mandated to be here to do business, we are mandated to fight austerity and the impact of Tory cuts on frontline public services and on vulnerable people.
“We are also mandated to negotiate if negotiations arise and that is what we intend to do. We intend to deliver on that mandate.
“It is up to others to explain, I suppose, what the effect their actions will have on our ability collectively to deliver for communities and to deliver for people who elected us all to this institution...
“We are not going to allow ourselves to be distracted and we intend to continue with that business.”
TUV leader Jim Allister, an arch critic of power-sharing, slammed the DUP response as “weak” and said few would notice if the Executive did not meet.
He said: “An Executive that doesn’t work and cannot deliver isn’t going to meet. Big deal.”
Meanwhile, Ukip leader David McNarry described it as a “grim day”.
He called for the Assembly to get back in touch with the needs of the public.