Security forces erected razor wire to prevent the protestors from entering the square.
Violence erupted on Sunday when anti-riot police used batons to break-up a demonstration by an estimated 2,000 people in Sanaa, injuring one woman and arresting 10 people.
The protestors, who are mainly students and civil society activists, want Saleh to follow the example of Egypt's Hosni Mubarak and resign. He has been in power for 32 years.
In Bahrain, there are reports of small-scale clashes during protests by the country's Shia majority. Demonstrators had called for a 'Day of Rage' on Monday along the lines of those organised in Egypt. At least 14 people were injured in clashes overnight and on Monday.
Meanwhile, Iranian police have blocked access to the house of opposition leader, Mir Hossein Mousavi, to stop him taking part in a rally in support of Arab revolts. According to his website, police vans and vehicle were sent to the alley where Mousavi and his wife live. The report also said all telephone lines at the house including mobile phone connections had been cut.
And Arab states are to hold their annual summit in the Iraqi capital, Baghdad, on 29 March. This wil be the first summit of the 22-member arab league since the resignation of Mubarak.