Wail Nawara - assistant to jailed party leader Ayman Nur - said on Friday that he and Ihab al-Khuly, Nur's lawyer and the party assistant secretary-general, were attacked on Thursday evening.
The Interior Ministry media office referred questions about the incident to the office of the public prosecutor, which is closed on Fridays.
Nawara said he had received several warnings in the last two weeks from sources connected to Egyptian security.
"Fifteen thugs in white training suits, accompanying seven men wearing plain civilian suits but armed with handguns, stormed the hall, interrupted the speaker and started insulting the panel," a statement from the party said.
Several Egyptian groups have
called for constitutional reform
"When Nawara attempted to ask them to be seated ... about 10 of the thugs attacked him, threw chairs, glasses and cups at him then got closer and started to beat him on the head, chest and back. They smashed his spectacles and attempted to strangle him with his own tie, then scarf," it added.
Police, who are posted at the door of every large Egyptian hotel, did not turn up to help for 60 minutes, Nawara claimed.
Nur was stripped of his parliamentary immunity in late January and detained for questioning about allegations that his party submitted forged documents when it applied for legal recognition last year. He is on hunger strike.
The liberal party says the allegations have been fabricated to punish Nur and his party for their vocal advocacy of constitutional change which would make it more difficult for President Husni Mubarak to stay in power.
The United States, which insists it wants to see democracy in the Arab world, has said it has "very serious concerns" about the Nur case.
"The last threat warned of beating and bodily harm if 'this nonsense about the constitution' was not stopped"
Assistant to Ayman Nur
Nawara said the party was holding a conference at the Pyramisa Hotel on constitutional change to show solidarity with Nur, who began his hunger strike on Tuesday.
He said he had received several warnings over the past two weeks from sources close to Egyptian security. Some had suggested that he leave the country, he said.
"When the 'friendly' warning was not heeded, a clear threat was delivered that if Nawara does not leave or shut up, he may be framed and accused of rigged charges. The last threat warned of beating and bodily harm if 'this nonsense about the
constitution' was not stopped," the al-Ghad Party statement added.