text: They said they had received an anonymous threat, two days after the killing of five police officers.
  A nearby parking lot was searched for a "suspicious person" but no-one was found. SWAT officers were earlier deployed at the main building.
  On Thursday, five white police officers were shot dead by a black man, Micah Johnson, during a protest rally.
  The march was against the killing of black men by police. Two deaths this week have led to nationwide protests.
  Dallas police said its officers - using dogs - searched the parking lot but the hunt turned up nothing.
  In a statement earlier on Saturday, the police said: "The Dallas Police Department received an anonymous threat against law enforcement across the city and has taken precautionary measures to heighten security."
  An armoured vehicle was moved close to the main HQ in central Dallas and heavily armed officers were seen nearby, according to the Associated Press news agency.
  But it added that members of the public were still able to walk about freely around the building.
  The police asked media to stop all live feeds around HQ "for the safety of our officers", the BBC's James Cook reports.
  The shooting happened late on Thursday during the protest march.
  Johnson, who was himself killed during the assault, supported black militant groups who encouraged violence against police.
  Dallas police chief David Brown said Johnson had told a negotiator that he had wanted to kill white people, especially white police officers, because he was angry about the recent shootings of black men by police.
  The attack came after the police killings of Philando Castile in Minnesota and Alton Sterling in Louisiana.
  Earlier on Saturday, President Barack Obama said the US was "not as divided as some have suggested" in the wake of the shootings involving African-Americans.
  He said Americans of "all races, all backgrounds", including many of those who were protesting, were outraged by the Dallas killings.
  As well as the five police officers killed, another seven were injured on Thursday. Two civilians were also hurt.
  Johnson, 25, who officials say acted alone, was killed by remotely detonated explosives that were sent into a car park where he had taken refuge after the shootings.
  He was a member of the US Army Reserve from 2009 to 2015 who had served in Afghanistan.
  Bomb-making material, rifles and a combat journal were found in his home in the Dallas suburb of Mesquite.
  A number of gun attacks involving police officers and civilians have occurred in other parts of the US in the aftermath of the deaths in Minnesota and Louisiana.
  Leaders of the Black Lives Matter organisation have condemned the Dallas killings but say planned marches, including a "Weekend of Rage" in Philadelphia, will go ahead.
target: Dallas police have given the all clear, hours after security levels were raised at their headquarters in the city.