Scalia is scheduled to participate in a "conversation" about civil liberties.
The ACLU said the conference has attracted more than 1,500 members.
"This is our democracy, our America, and we are here in Washington to reclaim it," said ACLU Executive Director Anthony D. Romero. He said that in the past five years, the Bush administration has had no respect for the rule of law.
Conference events began Sunday and will continue through Tuesday.
Issues to be tackled include racial justice, national security and abuse of power, the ACLU said. Several events will also feature young ACLU clients.