At least three people were stabbed in a scuffle between members of the Ku Klux Klan and counter-protesters near Anaheim, California, police said.
One of the victim is believed to be in a critical condition.
Four people were arrested, including one Klan member who is accused of the stabbing, said Sergeant Daron Wyatt of the Anaheim Police Department.
Three counter-protesters were taken into custody after stomping a KKK member on the ground.
Sergeant Wyatt said the altercation took place as soon as several Klan members arrived at the park in Anaheim, about 35 miles southeast of Los Angeles, for a planned rally there.
"As soon as they got out of their vehicle, immediately they were attacked by counter-protesters and this caused a melee down the block," he said.
Sergeant Wyatt said at least three counter-protesters and possibly a fourth were stabbed in the ensuing confrontation.
He stated that the most seriously wounded of the victims was taken to a local hospital, where he is listed in a critical condition.
The Klan, known for their white robes and peaked hoods often worn by its members, has a long history of violence toward African-Americans, Jews and other groups.
Several witnesses said that a peaceful counter-protest had been under way for about three hours in the park a mile north of Disneyland when the Klansmen arrived in a black vehicle.
British actor David Harewood has said that generations of black actors have been forced to head to the US because of a lack of roles in the UK.
The star of drama Homeland said there was now a “flood” of young black British talent coming to America as a result of the issue.
He said that he had been offered exciting stage parts in England - but not the TV work that can hugely boost careers.
“I’m both encouraged and worried by the amount of young black actors coming here,” he told ITV News correspondent Nina Nannar.
It’s great to see them come but also I’m wondering what’s going on in England that they are having to leave.
US sanctions imposed on Iranian entities as punishment for a recent ballistic missile test are illegitimate, the Islamic Republic has said.
Foreign ministry spokesman Hossein Jaberi Ansari said Washington's profitable sales of weaponry to the Middle East undermined the sanctions, which deny 11 companies and individuals access to the US banking system.
"The US sanctions against Iran's ballistic missile programme ... have no legal or moral legitimacy," he said.
"America sells tens of billions of dollars of weaponry each year to countries in the region. These weapons are used in war crimes against Palestinian, Lebanese and most recently Yemeni citizens," he said.
The dispute came after international nuclear sanctions were lifted on Iran for dramatically cutting back its nuclear programme and dumping tonnes of uranium in return for a lifting of crippling sanctions.
US president Barack Obama said the west had “cut off every single path” that Iran could have used to build a nuclear weapon through the landmark deal.
The release of a US journalist and three other dual citizens by Iran shows the extent to which relations have improved between the former foes, a Middle East analyst has said.
Dr Sanam Vakil said the releases under a bilateral prisoner swap had been a "surprise addition" to a landmark nuclear deal that saw international sanctions on Iran lifted last night.
"Its a sign of the confidence building that has come out of the nuclear agreement," she said.
Neither Iran nor the United States have had diplomatic relations for well over three decades - there's a lot of tension and mistrust.
But its sign that this is an administration in Iran that is seeking to build bridges and in areas of mutual interest there can be cooperation.
The Americans who left Iran after being freed in a prisoner swap deal have arrived in Germany, a US official said.
The swap comes amid the lifting of international sanctions against the country.
Three of four US citizens freed by Iran under a prisoner swap deal yesterday have landed safely in Switzerland.
The trio are believed to be Jason Rezaian, the Washington Post's Tehran bureau chief, Saeed Abedini, and pastor from Idaho and Amir Hekmati, a former Marine from Michigan.
The Swiss foreign ministry confirmed that the trio had arrived in Geneva, adding they were believed to have already left on the next leg of their journey back towards home soil.
A fourth prisoner, Nosratollah Khosravi-Roodsari, was also freed by Iran yesterday but is understood not to be on the flight.
US president Barack Obama said the west had “cut off every single path” that Iran could have used to build a nuclear weapon through a landmark deal.
The UN yesterday confirmed that Iran had dramatically cut back its nuclear programme and dumped tonnes of uranium in return for a lifting of crippling sanctions.
Mr Obama said the world was more secure under the diplomatic leap, which will see Iran subject to intensive ongoing inspections.
"If Iran tries to cheat, if they try to build a bomb covertly we will catch them," he said.
The bottom line is this: whereas Iran was steadily expanding its nuclear programme, we have now cut off every single path that Iran could have used to build a bomb.
He also hailed the talks as opening a "unique opportunity" for diplomatic negotiations, resulting in the release of four of its citizens held by Iran under a prisoner swap.
A Swiss plane carrying US prisoners freed by Iran on Saturday as part of a prisoner swap deal, and some family members has left Tehran, a US official has said.
"Yes, the plane has left," the official told Reuters news agency.
Jason Rezaian, the Washington Post's Tehran bureau chief, Saeed Abedini, and pastor from Idaho and Amir Hekmati, a former Marine from Michigan are believed to be heading to Switzerland.
Friends and colleagues at The Washington Post are elated by the wonderful news that Jason Rezaian has been released from Evin Prison and has safely left the country with his wife, Yeganeh Salehi
A fifth prisoner, American student Matthew Trevithick, was released separately on Saturday, the official said.
It comes as international sanctions against Iran were lifted after its government met all of its obligations under a deal agreed last July to curb its nuclear programme.
- Update: Nosratollah Khosravi-Roodsari was also initially reported to have been on board the plane but this was later clarified as not the case by a senior US official.
Iran's President Hassan Rouhani has hailed the lifting of international sanctions against his country as part of a nuclear deal as "a golden page in Iran's history".
Addressing parliament and presenting the draft budget for the next Iranian fiscal year, President Rouhani said the deal had "increased national confidence".
He added that the agreement would be "a turning point" in Iran's economy which has been struggling since international sanctions were introduced in response to Tehran's nuclear programme.
He called for economic reforms and less reliance on oil revenues in the post-sanctions era.
Iran is now expected to be able to access roughly $50 billion out of a reported $100 billion in holdings it has abroad.
Britain's Foreign Secretary Philip Hammond has welcomed the lifting of international sanctions on Iran and urged British firms to take advantage of the new opportunities that would open up as Iran was brought in from the cold.
The International Atomic Energy Agency (IAEA) in Vienna confirmed the government in Tehran had met all of its obligations under a deal last July to curb its nuclear programme in return for the lifting of sanctions.
Mr Hammond said the findings of the IAEA inspectors had confounded sceptics who said that Iran would never give up on its nuclear programme.
The deal paves the way for Iran to resume oil exports while recouping around 100 billion dollars in frozen assets.
The nuclear deal with Iran, in which Britain played a major role, makes the Middle East and the wider world a safer place.
Years of patient and persistent diplomacy, and difficult technical work, have borne fruit as we now implement the deal.
I hope British businesses seize the opportunities available to them through the phased lifting of sanctions on Iran. The future is as important as the landmark we've reached today.