It is fitting that on international human rights day the world turned this week to South Africa and Soweto, where thousands gathered to celebrate the life of one of the greatest humanitarians ever.

As we saw flags wave in the rain and tributes pour from world leaders, we all had cause to reflect on one man's journey from an underground liberation army, to 27 years a captive, and then to the Presidency of South Africa.

Mandela led millions out of state-sponsored subjugation towards democracy, human rights, and an inclusive multicultural nation. He is the most powerful, contemporary testament to the transformational power of politics. Not only can it right great wrongs in our society, but good politics can inspire a wholesale change in the attitudes of society too. And to that end, we can learn a lot from him in Scotland, not least when it comes to tackling a severe human rights abuse taking place in our communities today.

Walk down any street in Scotland and ask if slavery exists here and you will be met with disbelief. To the vast majority of Scots, slavery belongs to a murkier past, confined to history books on Wilberforce. Yet every four days another victim of human trafficking is recovered from our streets. They are found in brothels where they are held for sexual exploitation; in flats where they are forced to grow cannabis; and even in the restaurants or nail bars that make up our legitimate economy.

Yet despite hundreds of victims being identified in Scotland in recent times - with a 3% increase last year - and doubtless many others not spotted - still we have only secured three convictions for human trafficking in over a decade of criminal law against it.

The continuing reality is that we are failing some of the most vulnerable in our society: people suffering unimaginable human cruelty and manipulation on Scottish soil. In 2011, the Equality and Human Rights Commission reported its Inquiry into Human Trafficking in Scotland. This laid bare our failure to adequately protect victims and punish traffickers, and it set out a road map for change.

That report, and its 2013 follow-up, made high-level recommendations, including the need to fundamentally reform our laws around human trafficking, but not just in terms of the crime but also that any fit-for-purpose legislation must place victims' front and centre through statutory rights to assistance and recovery; underpinned by an independent decision making system unfettered by immigration concerns; with rights of appeal for those deemed not to have been trafficked.

The reports did recommend that any legislative change must start by introducing one clear definition of the crime, moving beyond the confusion of two offences that reflects the unsatisfactory, piecemeal treatment of trafficking in Scots law. This confusion would not be so serious where it not, as the EHRC Inquiry reported in 2011, an impediment to the police and other frontline services being able to spot trafficking when they encounter it. No wonder that Police Scotland's 25,000 leaflets to front-line officers use the definition we recommend than the two offences presently in Scots law.

And so, in September this year the Human Trafficking (Scotland) Bill was launched. It has been welcomed as the "most innovative and comprehensive piece of anti-trafficking legislation in the world" by Dr. Anne T Gallagher, one of the world's foremost experts in human trafficking recognised as such by Hillary Clinton and the USA State Department. Closer to home it has received the backing of many, including most recently, Scotland's finest legal minds at the Faculty of Advocates.

We believe the Bill harnesses the powers we have in Scotland to make our country the most unsafe place in the world for traffickers. However, it will only pass if the Scottish Government chooses to show the necessary leadership and dedication to allow the Bill to be heard in the Scottish Parliament. They have a choice to make.

Tomorrow, at Westminster, the Conservative Home Secretary Theresa May will launch a Modern Slavery Bill, and although we don't yet know precisely what is in it, all the mood music is that it will be a short, focused Bill e.g. not comprehensive, and it won't reflect the best in international criminal and human rights law, nor give statutory recognition to or rights for victims of human trafficking.

The choice for Scotland seems deceptively simple: it is, shall the Scottish Parliament, using all its current powers, take responsibility for Scotland's slavery by legislating for a comprehensive Human Trafficking (Scotland) Bill? Or does Scotland seek to accept what the Conservative-led Coalition offer at Westminster?

We think the decision should be clear: people in Scotland will expect its Parliament and Government to use the powers they have for victims, in criminal law, policing, and in other frontline services. These are the competences the Scottish Bill utilises; they are the powers the Scottish Parliament was created for; they are the powers that can effect change on slavery. This is crime not immigration.

The nature of human trafficking in Scotland; its impact on victims; merit comprehensive legislation. Trafficking is one of the great wrongs in our society. But it is embedded in how we live. So we must recognise that fact and enact the kind of long-term, generational approach only comprehensive legislation enables: that is if we are serious to move from concern to active condemnation of slavery.

We ask the Scottish Government to be bold, to practice the kind of transformational politics of Mandela and make a difference to those in slavery. We have world leading legislation built in Scotland and ready to be made into law. We can and we must seize the opportunity to make it so.