Young people in knit hats and jeans scurried around Thursday wielding brooms and trash bags, moving mountains of sleeping bags, backpacks and jackets out of the way...While demonstrators tried to tidy up the park Thursday — at one point a power washer was obtained, but there was no water available to pump through it — Scott M. Stringer, the Manhattan borough president, called on City Hall officials to try to reach a deal with the protesters. And the New York Civil Liberties Union issued a statement, saying: “The city must not use the cleanup as a pretext for mass arrests. To do so would be a violation of the spirit of the First Amendment and the spirit of dissent.”