July 28, 2010  -- Arizona's tough new immigration law was just hours away from taking effect when a federal judge issued an injunction today blocking key portions of the law from being enforced.

Among the provisions U.S. District Judge Susan Bolton put on hold are the "reasonable suspicion" section that would allow police to arrest and detain suspected illegal immigrants without a warrant and a provision making it illegal for undocumented day laborers to solicit or perform work.

Bolton also stayed part of the Arizona law requiring immigrants to carry federal immigration documents.

"There is a substantial likelihood that officers will wrongfully arrest legal resident aliens under the new [law]," Bolton ruled. "By enforcing this statute, Arizona would impose a 'distinct, unusual and extraordinary' burden on legal resident aliens that only the federal government has the authority to impose."

Some aspects of the measure, listed as SB 1070, will take effect Thursday as planned. It will become a crime for state officials to interfere with or refrain from enforcement of federal immigration laws. It will also be illegal to pick up and transport day laborers across the state, or to give a ride to or harbor an illegal alien. A vehicle used to transport an illegal alien can be impounded.

"The Court by no means disregards Arizona's interests in controlling illegal immigration and addressing the concurrent problems with crime including the trafficking of humans, drugs, guns, and money," Bolton wrote. But the court "finds that preserving the status quo through a preliminary injunction is less harmful than allowing state laws that are likely preempted by federal law to be enforced."

Lawyers for the U.S. Justice Department have argued the law interferes with the ability of the federal government to set and enforce national immigration policy.

"While we understand the frustration of Arizonans with the broken immigration system, a patchwork of state and local policies would seriously disrupt federal immigration enforcement and would ultimately be counterproductive," DOJ spokeswoman Hannah August said in support of today's ruling.

"It's a preliminary injunction so it's not final," said attorney Linton Joaquin with the National Immigration Law Center, which is party to one of the lawsuits challenging the law. "But the judge showed the most egregious provisions are pre-empted by federal law."

"It's good news for everybody," said Ali Noorani of the National Immigration Forum. "For now, all residents of Arizona will remain protected under the law."

More stories from ABC News' special series "Out of the Shadows: Illegal Immigration in America"

Supporters of SB 1070 Say Fight Will Continue

Supporters of the law were disappointed at today's ruling but pointed out that the blocked provisions could still take effect after the court resolves legal questions surrounding them.

"Today the Federal Government got relief from the courts to not do their job," said Gov. Brewer. "We knew regardless of what happened today of course one side or the other side was going to appeal. So this begins the process. This is an injunction. They haven't heard really the merits of the bill."

Steven Camarota of the Center for Immigration Studies, which favors the Arizona law, said today's decision was not unexpected. "The law had a lot of safeguards built into, but ultimately the judge didn't believe that Arizona could implement the law in an unproblematic way. This is strange because it assumes that, but there hadn't been any cases of misuse of the law."

Arizona Senators John McCain and Jon Kyl also expressed disagreement with the court's opinion and called the federal lawsuit against Arizona a "waste of taxpayer resources."

"The Obama Administration should have focused its efforts on working with Congress to provide the necessary resources to support the state in its efforts to act where the Federal government has failed to take responsibility," they said in a statement.

Judge Bolton did not rule today on various requests by lawyers for Arizona and Gov. Brewer to dismiss the various lawsuits against SB 1070. She also still has to decide on other preliminary requests to block parts of the law.

What's Next in Legal Fight OverArizona Law?

"After a preliminary injunction there is a trial on the permanent injunction," said New York Law School professor Lenni Benson.

The judge "can dissolve it later if, after greater consideration or development of more facts and time to assess the law, the judge thinks the law goes the other way."

The legal process is expected to drag on for months, after which, legal experts say, parties may still appeal parts of the decision they don't like. The debate over SB 1070 could reach the U.S. Supreme Court.

"This is a significant decision, and the litigation, as it proceeds to a final decision...and probable appeal, has potentially historic dimensions," said Hiroshi Motomura of the UCLA School of Law.

"Almost all state and local efforts to address immigration outside the law in this generation have focused on areas of traditional state regulation, such as employment and housing," Motomura said. "SB 1070 much more clearly is a direct attempt to address and regulate immigration itself. This is why the federal government had to take action in this case, in order to maintain its constitutional position as the exclusive authority on who is let into the United States, who must leave, and ultimately, who is or can become an American."

Immigrants Have Been Bracing for Arizona Law

Meanwhile, law enforcement officers across Arizona are preparing to enforce the measures not blocked today.

"We're not engaged in [racial profiling] now in all types of crimes we enforce. So what's the difference whether you enforce another crime, which is the illegal immigration? So we are very comfortable" enforcing the law, Maricopa County Sheriff Joe Arpaio said on "Good Morning America."

Many immigrant families have begun fleeing the state afraid of the new law's impact, and some activists are even encouraging immigrants to check tail lights on their cars, not to travel in large groups, and remove the Catholic rosaries from their rear-view mirrors.

Erika, a 23-year-old undocumented Mexican student living in Mesa, Ariz., since 1998, says her family is relieved with today's court ruling.

"Yesterday I went to bed really depressed," she said. "This morning, knowing this, everything came back. The hope, the faith, knowing that those prayers are really working."

"This means the community won't be as afraid to go with their friends, go out to the store," said Nicole Torres, who was participating at church vigil outside Phoenix after the ruling. "They hopefully will still feel safe enough to call the police now that they will no longer require you know checking of documentation."

Rosario Peralta, a legal immigrant who runs a south Phoenix grocery store, said some of the frightened families that had planned to move out of the state appear to be rethinking their decisions.

"I was worried about my customers..business was slow, but it's going to get better," she said.

The Arizona immigration law, which passed in late April, has attracted international attention and sparked protests around the country. It faces legal challenges in six lawsuits, in addition to the one filed by DOJ.

Sixty percent of Americans support the Arizona law, according to the most recent ABC News/Washington Post poll. But fewer -- 46 percent -- think the states should have power to make and enforce their own immigration laws, one criticism of the Arizona statute.

ABC News' Lauren Pearle, Jessica Hopper and Barbara Pinto contributed to this report.