Charles Taylor has denied all charges
Mr Taylor said his trial would not be fair because he only had one defence lawyer. His counsel walked out, defying the judge's order to stay seated.
Mr Taylor is accused of backing rebels in Sierra Leone who killed and maimed thousands of civilians over 11 years.
It is the first case of its kind against a former African leader.
Proceedings have been broadcast live on four giant screens in Sierra Leone's capital, Freetown.
HAVE YOUR SAY Yes. Let the proceedings begin! However, where are Taylor's accomplices?
Tito
Do international courts work?
Judge Julia Sebutinde ordered the trial to continue without Mr Taylor, amid intense protests from his lawyer, Karim Khan.
Mr Khan then left the court, saying he was not in a position to represent his client without further instruction from him.
After nearly one hour of wrangling, the prosecution began opening statements.
Proceedings at the UN-backed Special Court for Sierra Leone are expected to last between a year and 18 months, and the UK has offered to imprison Mr Taylor if he is convicted.
The former Liberian leader has been indicted on 11 charges of war crimes, crimes against humanity and violations of international humanitarian law over his alleged role in the brutal civil war in Sierra Leone.
What people in Sierra Leone and Liberia think of the Taylor trial
In pictures
Mr Taylor denies them all.
Human rights campaigners hope the trial will send a signal that nobody can escape punishment for atrocities, including heads of state, although some would have preferred him to have been tried at home by his countrymen.
However fears that Mr Taylor may still be able to mobilise a militia to attack a court in Sierra Leone were among the reasons cited for the trial being moved away from West Africa.
'Not fit for purpose'
Judge Sebutinde repeatedly pressed Mr Khan on the failure of his client to appear.
"He has not thumbed his nose at the court," said Mr Khan before producing a letter in which Mr Taylor said he felt he would "not receive a fair trial at the Special Court at this time".
CHARLES TAYLOR CHARGES Acts of terrorism (WC) Murder (CAH) Violence to life, in particular murder (WC) Rape (CAH) Sexual slavery and violence (CAH) Outrages upon personal dignity (WC) Violence to life, in particular cruel treatment (WC) Other inhumane acts (CAH) Use of child soldiers (VHL) Enslavement (CAH) Pillage (WC)
CAH: Crime against humanity
WC: War crime
VIHL: Violation of international humanitarian law
Taylor - the trial begins
While appearing to accept the court's jurisdiction, he said it was not "fit for purpose". He added he would not appear until "adequate time and facilities are provided".
"I cannot take part in this charade that does injustice to the people of Liberia and the people of Sierra Leone," he said in the letter. "I choose not to be a fig-leaf of legitimacy for this process."
Should circumstances change, he said, "I stand ready to participate in such a trial and let justice be done and for those who have suffered far more than me in Liberia and Sierra Leone."
The charges against Mr Taylor include terrorising the civilian population, murder, sexual violence, physical violence, using child soldiers, enslavement and looting.
The BBC's David Bamford says the trial hinges on determining the degree to which Mr Taylor helped start, prolong and deepen the civil war in Sierra Leone.
TAYLOR TIMELINE 1989: Launches rebellion 1991: RUF rebellion starts in Sierra Leone 1995: Peace deal signed 1997: Elected president 1999: Liberia's Lurd rebels start insurrection to oust Taylor June 2003: Arrest warrant issued August 2003: Steps down, goes into exile in Nigeria
Profile: Charles Taylor Q&A;: Trying Taylor
Mr Taylor started Liberia's civil war in 1989 and became one of a number of warlords competing for control in the West African country.
He later emerged as Liberia's most powerful politician and won the 1997 presidential election that ended the war there.
Meanwhile in 1991, one of Mr Taylor's comrades-in-arms, Foday Sankoh, started his own rebellion in Sierra Leone.
The prosecution claims Mr Taylor provided the Revolutionary United Front (RUF) leader with training, money, arms and ammunition to start the rebellion and even lent him fighters to take part in the initial attack.
The RUF became notorious for hacking off the hands and legs of civilians during their decade-long war.
It is alleged that Mr Taylor shared a common plan with the RUF's commanders to gain power and control over Sierra Leone, so he could gain access to its diamonds and have a government in Freetown that would support his aims.
However, the rebellion in Sierra Leone collapsed. Its war crimes court indicted the rebel leaders and Mr Taylor as well. Mr Sankoh died in 2003.
That year, Mr Taylor himself lost power in Liberia after rival militias rose up and forced him into exile in Nigeria.
He was deported by Nigeria last year in controversial circumstances and flown to The Hague to await his trial.
Former Liberian president Charles Taylor, who is facing charges of war crimes for his role in the Sierra Leone Civil War, has decided to boycott the proceedings at The Hague under the auspices of the Special Court for Sierra Leone. He faces 11 charges of war crimes, all of which he denies.

Taylor claims that his trial will not be fair as he only has one defense lawyer. "He has not thumbed his nose at the court," said Karim Khan, his counsel, before producing the letter in which Taylor said he would "not receive a fair trial at the Special Court at this time."

Chief Prosecutor Stephen Rapp disagrees with Taylor's assessment. Taylor, he says, has been assigned a lawyer, a special investigator and funds. "Everything that can be done is being done," Rapp told the Special Court.