* Group says wants to erase borders cross region
* Analysts split on influence of renamed Islamic State
* Leader Abu Bakr al-Baghdadi named "Caliph"
BEIRUT, June 30 (Reuters) - Militant Islamist fighters held a parade in Syria's northern Raqqa province to celebrate their declaration of an Islamic "caliphate" after the group captured territory in neighbouring Iraq, a monitoring service said.
The Islamic State, an al Qaeda offshoot previously known as Islamic State in Iraq and the Levant (ISIL), posted pictures online on Sunday of people waving black flags from cars and holding guns in the air, the SITE monitoring service said.
The Islamic State says it wants to erase national boundaries from the Mediterranean to the Gulf and return the region to a medieval-style caliphate.
Some analysts say the group is a credible threat to frontiers and is stirring regional violence while others say it exaggerates its reach and support through sophisticated media campaigns.
The group renamed itself and proclaimed its leader Abu Bakr al-Baghdadi as "Caliph" - the head of the state - on Sunday.
It also called on factions worldwide to pledge their allegiance, in a direct challenge to regional leaders and to the central leadership of al Qaeda, which has disowned it.
Pictures were posted on Twitter on Sunday and appeared to have been taken following the Islamic State's announcement. Reuters could not immediately verify the contents of the pictures independently.
"NOW WE ARE IN IRAQ"
The hardline Sunni Muslim group has been trying to build strong tribal support in its Raqqa bastion, the only provincial capital in Syria under rebel control.
Islamic State also controls other areas in northern and eastern Syria and across the frontier into Iraq, where it has advanced towards Baghdad from the northern city of Mosul, which it captured on June 10.
Islamic State also released a video called "Breaking of the Borders", promoting its destruction of a frontier crossing between the northern province of al-Hasakah in Syria and Nineveh province in Iraq, said SITE, which tracks militant websites.
Mosul, the country's largest northern city, is the capital of Nineveh. The video showed fighters from Islamic State killing Iraqi border guards.
"I say to the Islamic Ummah (community): Now we are in Iraq. Allah, glorified and exalted ... smashed these borders, the borders of Sykes-Picot, and now the Muslim can enter Iraq without a passport," the video said, according to a transcript.
"Sykes-Picot" refers to the division of the Ottoman Empire territories in 1916 by Britain and France.
The Sunni group follows al Qaeda's hard-line ideology but place more emphasis on anti-Shi'ite sectarianism, saying Shi'ite Muslims and other rivals are heretics deserving death.
In Syria, another monitoring group said the militants had recently crucified eight rival rebel fighters, leaving their bodies in a town square as a warning to others.
On Monday, pan-Arab daily al-Hayat showed a picture of bloodied, hooded men hanging from wooden stakes by their arms on a platform. One man was lying on the ground and all the men, dressed in civilian clothes, appeared dead.
It was not possible to immediately verify the picture, which showed the Islamic State's black flags placed on the edge of the square.
The Syrian Observatory for Human Rights, a group which tracks the violence, said Islamic State fighters crucified the men the town square of Deir Hafer in the Aleppo province on Saturday for being rival rebels.
Such infighting has killed around 7,000 people in Syria this year and complicated the three-year uprising against President Bashar al-Assad.
(Reporting by Sylvia Westall, Editing by William Maclean and Andrew Heavens)