Tikrit, the hometown of former leader Saddam Hussein, lies 95 miles north of the capital Baghdad.
Iraqi PM Nouri Maliki vowed to fight back against the jihadists and punish those in the security forces who fled offering little or no resistance.
The insurgents are from the Islamic State of Iraq and the Levant (ISIS).
ISIS, which is also known as ISIL, is an offshoot of al-Qaeda.
The Islamist group controls considerable territory in eastern Syria and western and central Iraq, in a campaign to set up a Sunni militant enclave straddling the border.
There were also reports on Wednesday of fighting further south, in Samarra, 70 miles north of Baghdad.
Separately, at least 21 people were killed and 45 hurt by a suicide bomber at a Shia meeting in Baghdad, police said.
As many as 500,000 people fled Mosul after the militants attacked the city. The head of the Turkish mission in Mosul and almost 50 consulate staff are being held by the militants, Turkish officials say.
Turkey’s foreign minister warned there would be “harsh retaliation” if any of its citizens were harmed.
The insurgents moved quickly south, entering the town of Baiji late on Tuesday.
There were heavy clashes reported in Tikrit, with dozens of insurgents attacking security forces near the headquarters of the Salaheddin provincial government in the city centre.
AFP news agency quoted police and witnesses as saying there was fighting at the northern entrance to Samarra.
Earlier PM Nouri Maliki vowed to fight back against the militants. He has asked parliament to declare a state of emergency.
In a live TV address, he said a “conspiracy” had taken place in Mosul and surrounding Nineveh province.
Nouri Maliki said he did not want to apportion blame for who had ordered the security personnel “to retreat and cause chaos”.
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