Relatives of 57 people killed in the Philippines' worst political massacre called on authorities Wednesday to speed up the suspects' trial as they marked the second anniversary of the murders.

Leaders of a feared clan are on trial over the killings but the proceedings have become bogged down, while another 100 suspects -- including other members of the Ampatuan family clan, policemen and soldiers -- are on the run.

Converging on a remote hill in the southern province of Maguindanao, where the massacre took place, families of the dead offered flowers and lit candles on a stone marker where names of the victims were etched.

Underscoring the fragile security in the area and the sensitivity of the case, police said they found and defused two bombs left by the side of the highway leading to the solemn site.

The devices were not strong enough to badly hurt or kill but were intended to send a warning, they said.

"It has been two years and there is still no justice. Some (suspects) have been arraigned (brought to court) but others are still out there," said Reginald Dalmacio, 28, brother of one of the victims.

While leaders of the Ampatuan clan, who are accused of orchestrating the killings, have been charged, their trial is still in its early stages and prosecutors fear it could take years more before anyone is punished.

Relatives of the victims have complained that witnesses are being killed, intimidated or bribed.

Philippine politics is well known for its violence, but the events of November 23, 2009 in a remote farming area in Maguindanao shocked the world.

Andal Ampatuan Jnr allegedly led a group of about 100 gunmen in stopping a convoy of cars carrying relatives of a rival political candidate, their lawyers and accompanying journalists, then massacring them.

Ampatuan Jnr allegedly led the killings because he wanted to stop Esmael Mangudadatu from challenging him in elections.

Thirty-two of the victims were journalists travelling with the convoy to witness Mangudadatu's wife, pregnant sister and other relatives lodge his candidacy to run against Ampatuan Jnr for the post of provincial governor.

Andal Ampatuan Jnr's father and namesake was at the time governor of Maguindanao and had been planning to install his son as successor.

Ampatuan Snr ruled Maguindanao for nearly a decade, building a reputation as a feared warlord who used a private army of a few thousand men to ensure he and his relatives won elections.

He ruled the province with the support of then-president Gloria Arroyo, who helped fund and legitimise his private army so it could be used as a proxy force against Muslim separatist rebels.

Ampatuan Snr and Jnr are among 64 people who are on trial in Manila, with a total of 93 suspects having been arrested.

Rights watchdog Amnesty International said the pressure was on President Benigno Aquino to speed up the "very slow wheels of justice".

"The government has to show that it has the ability to render justice in a massacre that constituted the world's worst-ever attack on journalists and the world's worst-ever election-related single incident," it said.

The US government said in a statement that the international community was watching.

"The prosecution of this case is seen by many around the world as demonstrative of the Philippines' commitment to upholding the rule of law and protecting human rights," US Ambassador to Manila Harry Thomas said.

Prosecutors said they were trying their best but the huge number of suspects and the stalling tactics of the defence lawyers had slowed proceedings.

The Philippine justice system is notoriously slow, with a trial taking an average six years to complete, even on less complicated cases, according to government data.

Victims' relatives are pinning their hopes on Aquino, who won elections last year in a landslide after promising to end the culture of impunity.

On Tuesday, they sued Arroyo for arming and supporting the alleged murderers.