Labor and the Democrats will move in the Senate today to set up an inquiry into the Government's Pacific solution and its claim that asylum-seekers threw children overboard late last year.

The inquiry, which will attempt to call navy witnesses and former defence minister Peter Reith and may even travel to Papua New Guinea to speak to the asylum-seekers, will report by May 16.

The agreement came as up to 3000 protesters rallied at Parliament House, in an event that was mostly peaceful but turned rowdy when Governor-General Peter Hollingworth arrived for the opening of Parliament.

More than 100 police and Australian Protective Service officers held the chanting crowd back, making one arrest, warning another protester and dragging several back into the crowd.

Darren Bloomfield, 34, was arrested for breach of the peace but released by the Magistrates Court after giving an undertaking not to return to Parliament House yesterday.

Labor frontbencher Carmen Lawrence was flanked by 10 colleagues in a show of support as she addressed the rally, telling the protesters that voters had not been offered a choice at the last election, but Labor had taken an important first step this week in looking for alternatives to detention.

"We can incarcerate asylum-seekers indefinitely in remote and inhospitable camps where brutal treatment goes unreported," she said. "Or we can ensure a regime where community release is a norm and people are not detained.

"There are alternatives and we must embrace them."

Dr Lawrence chose her words carefully after being warned by Leader Simon Crean that she must toe the party line, but she continued to push for an end to mandatory detention, stressing that Labor's blueprint only endorsed detention for initial checks and to stop high-risk people absconding, but made no mention of detention while claims were being processed.

She was supported on the stage by Labor backbenchers George Campbell, Anthony Albanese, Jann McFarlane, Jan McLucas, Duncan Kerr, Kelly Hoare, Julia Irwin, Maria Vamvakinou, Trish Crossin and Tanya Plibersek. Canberra Labor MPs Kate Lundy and Annette Ellis also joined the rally.

Their show of support came after Liberal MPs Petro Georgiou, Marise Payne, Christopher Pyne and Bruce Baird questioned Coalition policy this week. Ms Plibersek told protesters Australians would look back on the treatment of asylum-seekers with the same shame they now viewed the genocidal treatment of Aborigines.

"What a criminal failure of empathy that the leaders of this country could not say to that man who lost his daughters and his wife, you can go and bury those little girls," she said, referring to Iraqi refugee Ali Madhi, who lost his family at sea.

Federation of Ethnic Community Councils spokesman Sandy Santmyers said Australia had not even filled its "rather modest" target of 12,000 refugees a year.

"We join in voicing our fear of the growing divisiveness and hatred in the community that has raised the indignation in some instance of people around the world," he said. "Our message to Government: speed up the processing, fill the 1000 empty places and for God's sake listen to your advisers."

Greens Senator Bob Brown said, "If international law were to have penalties and if the Woomera centre were to be a receptacle for those who breach international humanitarian law, our Prime Minister and our Minister for Immigration would be behind the razor wire themselves."

Democrats Leader Natasha Stott Despoja said the Government would be judged as criminal because of the way it treated people in detention.

To set up their inquiry, Labor and the Democrats need the vote of Senator Brown and one vote from among Independents Brian Harradine and Shayne Murphy and One Nation Senator Len Harris.

Labor agreed to widen the inquiry beyond the child-throwing allegations, to cover the Government's Pacific solution, as requested by the Democrats. If agreed today, it will look at the agreements reached with Nauru and Papua New Guinea and the cost of keeping people there.

Democrats Senator Andrew Bartlett said he would push for the committee to visit Manus Island to interview detainees accused of throwing their children overboard, as well as calling navy personnel who witnessed the event and "anybody who got told something at some stage down the line", and the United Nations High Commissioner for Refugees.

Senator Bartlett will also move for a second, broader, inquiry that will look at conditions in camps on the islands as well as allegations surrounding the Woomera centre in South Australia.

Senator Brown will attempt to widen the inquiry to cover allegations the Government listened in on telephone calls with the Tampa last year.

Also yesterday, UN High Commissioner for Human Rights Mary Robinson welcomed the Government's decision to allow her envoy to inspect the Woomera detention centre, but said the visit should take place no later than May.